The Adult Legion series of stories were a part of "imaginary stories" that appeared often in the Silver Age Superman titles. These stories might easily be explained as events in parallel universes.
The Adult Legion first appeared in Superman #147 (1961) and continued sporadically through 1967. These adventures all happened in Superman's time, not Superboy's. Therefore, the Legionnaires had grown up as well.
Adult Legion tales were always questionable to mainstream continuity because they depicted events in the Legion's future that never came to pass, or seemed unlikely to occur as Legion history unfolded. For example, adult stories predicted that Light Lass and Timber Wolf would marry, and that Quantum Queen would become a Legionnaire; and that the Legionnaires would all change their names to "Man" or "Woman." One Adult Legion prediction did come true: the death of Chemical King.
For these reasons, it's safe to assume that these events took place in a (pre-Crisis) parallel universe. Of these characters, Saturn Queen was one of a handful of survivors to slip through the first Crisis and collapse of the multiverse (like Superman and Power Girl from Earth-2, Dark Angel of Earth-7, Alexander Luthor and Ultraman of Earth-3 and Superboy of Earth-Prime).
Adult Legion Chronology
|30th Century: Superman visits the now-adult Legion in the 30th century. The adult Legion is attacked by Douglas Nolan (twin brother of Ferro Lad), under the mental domination of the Legion of Super-Villains (1st chronological app.). Rokk Krinn (Cosmic Man) is married to Lydda Jath (Night Girl), Jo Nah (Ultra Man) married to Tinya Wazzo (Phantom Woman) — with twins — Ayla Ranzz (Light Lass) married to Brin Londo (Lone Wolf), and Tenzil Kem as President of Bismoll. It also shows statues of several dead Legionnaires: Chemical King, Power Boy, Quantum Queen, Reflecto, and Shadow Woman. Chemical King, Reflecto, and Shadow Lass eventually joined the Legion. NOTES: Quantum Queen was as a member of the Wanderers and Power Boy was a student at the Legion Academy. The events of Adventure #354-355 precede the Adult Legion's first print appearance because they still feature Saturn Queen (who later quit), and because their eventual modus operandi is to raid eras other than their own.||Adventure #354 (3.67)|
|30th Century: The Adult Legion and Superman fight the Legion of Super-Villains, which now includes Echo and Beauty Blaze. The Legionnaires are rescued by benevolent 30th century descendants of Lex Luthor and Mr. Mxyzptlk, who join the Legion. NOTE: In canonical Legion continuity, Echo briefly became a Legionnaire during the "Five Year Gap."||Adventure #355 (4.67)|
|20th Century: 1st app. in print: Legion of Super-Villains (Cosmic King, Saturn Queen, and Lightning Lord) to help him battle Superman. Superman is aided by the Adult Legion of Super-Heroes: Cosmic Man, Lightning Man and Saturn Woman and eventually by Saturn Queen, who turns against the other villains and vows to reform. NOTES: The Super-Villains tell Luthor they are from the 21st century, rather than the 30th century. As established in Legion v.2 #300 (6.83), the Adult Legion (and adult LSV) that appeared in various stories were from an alternate timeline and were not canonical as far as the Legion was concerned, even though some of the events predicted in the Adult Legion stories did come true. Evidence suggests, however, that the Adult Legion stories involving Superman and other 20th century characters (except for those specifically described as Imaginary Stories) were part of pre-Crisis Superman continuity, even if they were not the "real" future of the Legion. This story gives the first account of Lightning Lad's origin and the first appearance of the planet Korbal, where the Ranzz family got their powers. It also reveals that Lightning Lord is Lightning Lad's older brother, a fact later reflected in canonical Legion stories.||Superman v.1 #147 (8.61)|
|The Superman Revenge Squad exposes Superman to Red Kryptonite, causing him to experience a terrifying nightmare in which he is put on trial by his enemies, including the adult Legion of Super-Villains. NOTES: The Superman Revenge Squad's first chronological appearance, as the Superboy Revenge Squad, was in Superboy #94 (1.62).||Action #286 (3.62)|
|20th Century: Supergirl and Superman travel to 10 years in the Legion's future. Supergirl tries to fix Superman up with Saturn Woman (the adult Saturn Girl), only to find that she is already married to Lightning Man (the adult Lightning Lad). NOTES: This was the second appearance of the Adult Legion in print, and the first hint that Garth and Imra would eventually marry.||Action #289 (6.62)|
|20th Century: Cosmic Man and Lightning Man pose as Hercules and Samson to help Superman fool a criminal. This was the third appearance of the Adult Legion in print.||Superman v.1 #155 (8.62)|
|20th Century: Jimmy Olsen and Supergirl battle members of the adult Legion of Super-Villains, including Lightning Lord, Cosmic King, Chameleon Chief, and Sun Emperor. NOTES: Teenage versions of Chameleon Chief and Sun Emperor later debuted in Superboy #208 (1975).||Jimmy Olsen #63 (9.62)|
|20th Century: Saturn Woman and Proty II help Superman defeat the Superman Revenge Squad.||Superman v.1 #165 (11.63)|
|20th Century: Lex Luthor, Brainiac, and the Adult Legion of Super-Villains try to brainwash Jimmy Olsen into killing Superman.||Jimmy Olsen #87 (9.65)|
|Adult Legion: The Adult Legion and Superman fight the Legion of Super-Villains, which now includes Echo and Beauty Blaze. The Legionnaires are rescued by benevolent 30th century descendants of Lex Luthor and Mr. Mxyzptlk, who join the Legion.||Adventure #355 (4.67)|
|20th Century: The adult Brainiac 5 helps Superman and Batman defeat Xan, an alien who has taken Joseph Meach's place as the second Composite Superman. NOTES: Although the adult Legionnaires are from an alternate timeline and not the "true" future of the Legion, this story is part of pre-Crisis Superman and Batman continuity.||World's Finest #168 (8.67)|
|Imaginary Story: Bruce Wayne and Clark Kent are raised as brothers. Batman eventually moves to the 30th century and joins the Adult Legion. NOTES: This Imaginary Story is not technically part of pre-Crisis continuity.||World's Finest #172 (12.67)|
|20th Century: Superman, Supergirl, and the adult Brainiac 5 trick Lex Luthor into freeing Superman escape from a trap set for him by Mordru. Brainiac 5 mentions that the White Witch is a member of the Adult Legion.||Superman v.1 #213 (1.69)|
|THE DREAMS OF DOUGLAS NOLAN|
|Douglas Nolan succeeds his brother Andrew as Ferro Lad II. NOTES: In mainstream continuity, was a vision dreamt by Douglas Nolan. Three of the dreams in this issue reference the Adult Legion. This issue established that the events of the Adult Legion stories took place in alternate timelines, not the canonical Legion's.||Legion v.2 #300 (6.83)|
|The Adult Legion (Chameleon Man, Sun Man, Element Man, Karate King, Timber Wolf) battle the Fatal Five. NOTES: In mainstream continuity, this was a vision dreamt by Douglas Nolan. The living/deceased Legionnaires in this vision Hall of Heroes is consistent with other Adult Legion stories, plus Cosmic Boy, Saturn Girl, Lightning Lad, Matter-Eater Lad, Projectra and Shrinking Violet. Legionnaires wear a mix of costumes from their original and 1970s eras.||Legion v.2 #300 (6.83)|
|The members of the adult Legion (Wildfire, Ultra Man, Phantom Woman, Chemical King, Tyroc, Brainiac 5, Cham, Element Man, Superman, Sun Man) argue among themselves about how to deal with a Khund incursion. It leads to Superman resigning. The rule prohibiting married Legionnaires from serving is still in place. The former Dream Girl and Star Boy watch the Legion from home with their children. Some Legionnaires launch themselves into battle; Tyroc is killed. Karate Kid is memorialized on the wall. NOTES: In mainstream continuity, this was a vision dreamt by Douglas Nolan. The living/deceased Legionnaires in this vision Hall of Heroes is consistent with other Adult Legion stories, plus Karate Kid. Legionnaires wear costumes from their 1970s era.||Legion v.2 #300 (6.83)|
|THE CRISIS ON INFINITE EARTHS|
|Imaginary Story: "Whatever Happened to the Man of Tomorrow?" A group of Legionnaires, accompanied by Supergirl (years before her death), gives Superman a trophy of his final and greatest victory.||Superman v.1 #423 (9.86)|
|Imaginary Story: "Whatever Happened to the Man of Tomorrow?" The Legion's trophy gives Superman the clue to defeating Mr. Mxyzptlk. The adult Legion of Super-Villains appears on the scene to gloat, but the villains flee when they realize they may be injured.||Action #583 (9.85)|
|20th Century: Lightning Lord and Cosmic King are swept to the 20th century by the Anti-Monitor's antimatter wave. They battle alongside the 20th century's Lex Luthor and Brainiac in an army of super-villains.||Crisis #9 (12.85)|
|POST-CRISIS ADULT LEGION|
|21st Century: The Adult LSV make a final effort to save their timeline from the Crisis' destruction. They alter the 20th century origins of Superman and Batman and raise the two as their sons. Their changes result in a metahuman war that destroys this fragile, changed reality. The LSV escape back into the time stream to plot anew. NOTES: The Adult LSV that appears in this story have supposedly left the 31st century just before the original Crisis wiped away their timestream.||Superman/Batman #14-15 (1-2.05)|
|21st Century: The Superman of the "imaginary" timeline witnesses the destruction caused by the LSV and now sits in Metron's chair on the charred and barren planet Earth, destroyed by the LSV's tampering of the time stream He meets the quantum leaping Captain Atom, who challenges Superman to do something about it.||Superman/Batman #5 (3.04)|
|21st Century: The "imaginary" Superman allies with Darkseid, who is also angered by the LSV's time tampering. They tell Superman and Batman to go back and fix things, sending them back to prevent the LSV from murdering the Kents. The Adult LSV retool their plan and return to the 21st century with Echo and Beauty Blaze, allied with Ra's al Ghul. They are defeated again.||Superman/Batman #16-18 (2-4.05)|
|31st Century: Superman and Batman of Earth-0 return the LSV to their own era, to a point in time just prior to their original departure. They're remanded to the custody of the adult Legion of Super-Heroes. After Superman and Batman leave, this timeline falls to the antimatter waves of the first Crisis. The "imaginary" Superman returns to his wife and son just before this timeline ceases to exist.||Superman/Batman #18 (4.05)|
|21st Century: Following the Crisis, Saturn Queen finds herself a survivor of the old multiverse, living in the Earth-0 universe. She joins another alternate universe survivor — Ultraman of Earth-3 — in making their home in the shrunken city of Kandor. They take control of the city, until Supergirl and Power Girl come to challenge them. Supergirl eventually agrees to leave Kandor in exchange for information from Saturn Queen about the fate of her Kryptonian home, Argo City. NOTE: The creation of the new multiverse was fully revealed in 52 #52 (5.07). It is not specifically stated that Ultraman was the one from pre-Crisis Earth-3 or that of the antimatter universe. In the post-Infinite Crisis multiverse, the antimatter Crime Syndicate may have been destroyed, as hinted in JLA Secret Files (2004).||Supergirl v.5 #6-8 (4-9.06)|