Their designs are just ambiguous enough that you can just ignore their canon age.
I don't necessarilly get it for some characters (like Deku), and I'm disgusted by shota, but at some point you have to just let go and accept that people will just sexualize everything without it having any impact on who they would try and bang in real life. These are characters, they don't have lived experiences, they can't be damaged by this stuff., so yeah even if it's a bit iffy on principle there's a nuance to be had.
Heck, even when people do take their age into account and sexualize them, it also can be out of a fantasy developped from being frustrated when they were younger, out of a desire to have been able to explore their sexuality as teens with other teens. Fiction allows them to relieve some of that frustration by projecting. I know I read stuff set in high schools (mostly with characters that don't look as young as they're supposed to be) and it turns me on, not because I want to bang real life teens but because it plays with what I saw as impossible back then.
Just let live.