I don't see a problem with kids and teenagers on videogames, after all, kids and teenagers play videogames and will identify easier with characters of the same age.
A completely different issue is sexualized, young girls. I think you made a mess of OT by mixing both things that are completely different.
The first thing is not an issue at all, It is just a product of those games catering to different age demographics that skew towards kids/teens, that's the reason to almost always include characters in that age demographic. (This was the actual point of the OT isn't? why having child-like characters on Japanese games). I mean, anyone had a problem with Toon Link and Tetra in Wind Waker? I hope not, they are kids and they are having a colorful adventure (and sticking a master sword in the bad guy's forehead...)
I'm my point of view, is also Ok for teenagers having romantic relationships on videogames, since during adolescence is when most people have their first romantic endeavors, but that can be depicted nicely on the game, without need for scampy clothes and so. Something like Futaba scene in Valentine day (Persona 5), where they cuddle on the sofa and then is implied they spend the night together, and that's it. They don't need to show anything, and everything is handled fine. But yeah, on the same game Ann is treated like eye candy :(
Underage sexualized characters, that's a more troublesome topic that should be talked in a different setting than just children on videogames.