Board Thread:Questions and Answers/@comment-28156355-20160427223255/@comment-71.83.236.251-20160428031656

If a Servant is incarnated, yes. Gilgamesh used his potion of youth in Fate/Zero than aged into adulthood again. This is an extreme scenario, because Gilgamesh had an actual physical body that could produce its own prana / magical energy. Such a thing is normally impossible for a Servant. Another Servant that incarnated, 'Shirou Kotomine', also aged but his Noble Phantasm kept him youthful.

Servants are generally summoned into existence in their 'prime'. But its possible for them to assume forms that they didn't have in life. It's best not to think of them as just a human, but instead as a phenomenon, and their materialized body is a reflection of the phenomenon that is their legend. Basically I don't think a non-incarnated Servant will age physically.