I suspect when it comes to the way young characters are portrayed in anime doesn’t necessarily make them unrelatable. But for every anime that don’t dwell much on cliches or actually focus on other things like Cooking Master Boy, there’s bound to be anime that does. Another problem’s that whatever their age, usually on average there isn’t much truly mature teen characters.
Maybe I might be wrong in here as there are anime where teen characters actually do mature. But if most anime’s intended to be wish fulfillment, whilst this isn’t always the case to whatever extent, a truly adult female character’s mostly intermittent. There are likely anime where characters do try to care for/parent younger relatives. But my point’s that if you make them too mature, it feels off-putting.
Especially to people who’re used to expecting women and female characters to be this childish and incompetent that a genuinely strong, mature female would be astonishing and horrifying. There are big sister characters in anime. But the truly mature female characters seem rare at best. As in both non-stereotypical older women (both adult and senior) and truly mature young women.
I have a feeling a mangaka might agree with me here and depict a genuinely mature young woman caring for her family. My point still stands that if she’s shown to be actually mature, she’d seem odd to some people even if there are cases where it necessarily and inevitably happens. (There’s a Chinese girl whose parents are often away and she’s left with caring for herself and her brother a lot.)
Mature female characters do exist in anime but it gets outweighed by idealised teen girls and childish adult women that when coupled with sexualisation becomes off-putting to people.