[Insert a rule 34 character "content" ammount counter]
Chara (6859)
This is all I need to show you.
Now I must say, you are correct, hovewer, the internet decided that Chara is hot as hell if she 1: is actually a woman and NOT a genderless child just like Frisk; and 2: aged up to at least 18, usually with... interesting proportions.
I'll be honest and say that there is 100% at least 1 piece of "special" art about her that depicts her as under 18, but this isn't the case here as far as I can see (didn't even download the thing yet)
Basically: she isn't a child, at least not here specifically