There is a girl who hasn't aged well and I want to tell her.
What do you mean exactly in regards to her aging?
I'm going to tell her that she looks old.
It is wrong to judge people by their age. You should keep some opinions to your self because other people might find you mean and rude.
She shouldn't care about what I say.
Criticizing someone's look is mean and some people really get upset when they think they look older. There is no need for you to say this, as it's not nice and you will legitimately hurt their feelings.