Can a person, who doesn't have a beard, do tawbah for apostasy?

Yes, any person can do so.

Tawbah is between man and His Creator. At the time of Tawbah, an individual should be truly sincere, have a sense of remorse within him and promise Allah Ta?ala that to the best of one?s ability he will not return to that sin. After Tawbah, he should make a concerted effort to mould his life according to the injunctions and dictates of Islam.

and Allah Ta'ala Knows Best

Mufti Ebrahim Desai