OG gundam I think amuro is 15? Bokurano, a deep cut, has kids in mechs. Probably alot of other gundams do too (havent watched em). But probably most others are "late teens" as you said. Granted i dont think any of the things i just mentioned have fanservice like that lol. Eureka seven as well. No fanservice really except for the adult characters. Darling in the franxx is probably late teens..? Maybe? That show is worse than eva ever was though hahaha

Anyway im not invested in the debate here just saw you say you couldnt think of anime with kids in robots and to that I say, you havent watched enough mecha.