Really, really old.
Older than the oldest bit of kit you were ever issued, and then some.
You know that old bloke who sits in the corner of the bar, telling all and sundry that he was second man in on the balcony when he was one of THEM? Well, "ancient" means even older than him.
In military history terms, the Ancient period covers everything from the dawn of time up to the fall of the Roman empire in 476 AD.
This wiki page is a good start.