So, one of the things that I've pondered from time to time, is the idea of Kal-El remaining in the Legion after he took on the name of "Superman" as opposed to becoming inactive at some point. (I've always assumed he stopped joining the Legion's adventures once his parents died.)
I've always thought that the Legion would be a good place to show Kal's career from 18-22 or so. Possibly right up until the time the Justice League became a going concern.
I've always assumed that Kal joined the Legion at 15 or 16. (14 is a possibility, I suppose as Pre-Crisis Superboy started his career at age 8 or so.) At any rate, even if he joined the Legion at 14, he would have had about 4 years to be an active Legion member. (I'm pretty sure that Clark's parents died right when he starting college, so I'd say that he couldn't have been a member much past his high school graduation.)
In contrast, the rest of the Legion aged at least 10 years-- maybe more-- during Kal's time with them. To have the Legionnaires go from his peers to full-fledged adults (even if they did still call themselves "Boy, Girl, Lass, Lad, etc.) would have seemed pretty jarring to me. Giving Clark a membership that lasted until he graduated college would have made more sense to me ... and the artists could even have gradually shown Clark bulking up from his comparatively slender teenage build to the man he became.
(And one of these days I'm going to write my fanfiction idea about Clark's best friends in the Legion coming back in time to spend time with him so he's not alone on his first Thanksgiving or Christmas without his folks. My biggest problem with that idea is figuring out who his best friends in Legion actually are.)