Marriage Fiqh: Age is not an issue in Marriage:

In many different cultures age of bride and bride groom is taken as an important issue.Most of the cultures think that bride should not be too young .Similarly some people think that bride should not be elder than bridegroom.In Islam there is no such restriction.
Prophet salal laho aliehi wasalam married Khadija radi Allaha anha who was elder than her.He married Ayesha radi Allaha anha who was a young girl at time of marriage.Similarly Hazrat Sauda Radi Allahu anha belonged to age group of prophet salal laho aliehi wasalam.

The important issue is that bride should be physically and mentally mature.

Maturity is relative term which is achieved by different individuals at different ages.