Personal pronouns following the verb "to be" should be in the nominative case (I, he, she, we, they).
1. What age is her? (wrong)
2. What age is she? (correct)
However, in common spoken English this rule is often broken:
A: Who's that at the door?
B: It's me!
Technically, the above is wrong. But few people would say "It is I".
Because t