Elle est vieille mais elle n'est pas trop faible.
Yes it is long but i cant see why it's wrong.
Veille works, not sure why the rest doesn't
I think the second part translates to "and she is not too weak" which isn't exactly the same.
Why agee and nor vieux ?
"Vieux" is masculine. The feminine is "vieille".
Vieux is the masculine form. "Vieille" should work.