I know that the Imperial Space Marines are modified to be super human, but apart from violent death in battle do they age at all? Do they ever reach a point where their bodies are no longer fit for battle and they must set aside their battle armor?
In theory, they could. In practice, they don't. They don't really "age" in the sense you're asking about, and they're in much too great a demand to ever be allowed to leave, even if they wanted to.
Space Marines are practically immortal - that is, if they're not stabbed or shot or burned or dismembered or poisoned or in some other way acutely damaged, their bodies are modified to be capable of handling the diseases that kill most people and the slow, systemic breakdown that leads to normal "old age". However, since the Great Crusade and the Horus Heresy, no Space Marine has ever had a prospect of a life without those risks.
The current oldest Loyalist Space Marines is the Blood Angels Chapter Master, Dante, clocking in at over 1100 years. He's closely followed by the Space Wolves Chapter Master, Logan Grimnar, who has been the Great Wolf for over 700 years so he's presumed to be at least pushing a millennium.
Dreadnoughts "live" even longer, as exemplified by Bjorn the Fell Handed, one of the Space Wolves's Dreadnoughts, who is one of the last surviving Loyalists to have personally met the Emperor before he was interred on the Golden Throne, 10,000 years ago.
However, amongst the treacherous Chaos Space Marines, it's believed that some have lived at least since their creation 10,000 years ago. Some have possibly even lived longer, given the rather loose definitions of time in the Warp.