In general, this is a big problem with BGS games for me because I play adolescent girls and prefer early- to mid-adolescence (i.e., 18-19 is essentially an advlt and may legally be advlt in some cultures). As was said, BGS may have to deal with problems/uncertainties regarding legal stuff such as depicting killing/harming adolescents, or sixual content of certain kinds where such characters may be involved. They skirted the issue with Little Lamplight and I think they did an excellent job in that case, but that might not be possible in most contexts.
I use the shoujo race mod in my FO3 and FONV. Before the mod, I simply adjusted my own size using the console (but that's not as good/accurate as a mod, of course). In Oblivion, I made my own custom races so that the size and other characteristics would behave properly in the game world (setsize doesn't necessarily make the NPCs and game world realize your changed size, at least in Oblivion).
Some of the kids in FO3 seem to be early adolescents.
The PC is given to be 19, of course.
Bittercup might be adolescent but it's not stated for sure.
Veronica in FONV might be adolescent. I use a mod that makes her and Cass shoujo so they are both adolescents in my game.
In my FO3, I use a mod to make some other characters such as Brianna and Jenny shoujo. Also the BoS scribe.
These are just a few examples that might be helpful.