....yet, there have been times when they have been too young!
Here are those adventures!
The Menace of Dream Girl
Dream Girl applies for Legion membership, showcasing her powers of accurately dreaming the future (only a few days in advance), and, due to her beauty, quickly gets all the male Legionnaires on her side (earning the ire of the female Legionnaires). Dream Girl is accepted as a member, and studies the Legion constitution....but for what seems a nefarious purpose. First, sending Lightning Lass on a mission that removes her lightning powers (which is a removable offense of the Legion of SUPER-Heroes).
Fearing the ill will of the team, Dream Girl leaves the team (keeping track of Star Boy, who would later become involved with the seer from Naltor), everyone is reinstated to the Legion, with Lightning Lass now having new powers to reduce the mass of anything (as opposed to lightning powers). The battle with Time Trapper continues on and off for many issues after this....until....
The Menace Of The Super-Babies
Saturn Girl gathers Legion members to prepare them for a final battle with the Time Trapper, as Saturn Girl has been contacted by Glorith, who meets with the Legion at an abandoned amusement park in the 30th Century. Glorith shows the team her ring which can penetrate the Iron Curtain, and gets them to touch the hourglass, but, thanks to chemical mist from a nearby fountain, the Legion members present (Saturn Girl, Chameleon Boy, Invisible Kid, Ultra Boy, Matter-Eater Lad, Element Lad and Light Lass) into infants!
Brainiac 5 and Superboy follow the Trapper and the youth, eventually convincing him to restore the ages of the afflicted Legion members (and thanks to a mischievous baby Star Boy and the wisdom of Brainiac 5) are able to trap him behind his own Iron Curtain (for a time).
The Five Legion Orphans
It was Parent's Day, with the various members of the Legion celebrating with their parents. But, that excluded the orphans....Superboy, Brainiac 5, Mon-El, Element Lad and Dream Girl. The five, responding to an alarm on the planet Zinth, try to retrieve a crystal from a pond, but all five regress in age after exposure. They end up in an orphanage, and are all adopted by parents from the planet Baskh. Brainiac 5, whose intellect is unaffected by the de-aging, works on a cure for himself and his friends, eventually coming up with one.
...those are adventures for another time!