Can I tell a girl she didn't age well?
Why are you going to tell this girl that she didn't age well?
Because I think she deserves it and I don't care for her anymore.
Even if someone has lost their looks over time, it is inappropriate for you to point this out in a blatant way to them. Have some consideration for this girl's feelings and refrain from embarrassing her about her looks.
Nah, I'm going to do it anyways
Telling someone they did not age well is an insult and will make the person angry.