his/her age
Está en buena forma física para un hombre de su edad, Sr. Brown.
He’s in good physical shape for a man his age, Mr. Brown.
— Paddington 2 (2017)
Ella lo debería saber mejor a su edad.
She should know better at her age.
We're about the same age as him/her.
Tienes que tener en cuenta su edad.
You have to take their age into account.