She is small for her age

Proszę, jak będzie po polsku:

She is small for her age.

(ona) jest mała jak na swój wiek.


Whether or not you should use the pronoun 'ona' depends on the context. Generally, the pronouns 'on', 'ona', 'ono' are superfluous if it can be deduced from the previous sentences who (or what) you are referring to.