Hi Has - am I right in guessing you were on a beach in the south-east or maybe east anglia? There are many prehistoric stone and bone objects washed up around there, and even caught in trawlers' nets, from the land covered by the post-glacial flodding. It's called 'Doggerland' now and archaeologists are trying to map and record it properly. So first and foremost, the nearest county archaeologist, eg this one for Suffolk might be interested in a photo and some information:
http://www.suffolk.go...ironment/Archaeology/ = put something in the photo to give it scale, such as a ruler or a 2p piece.
It's impossible to judge its age with any accuracy without seeing it, but broadly speaking stone tools become smoother and more polished in finish towards the New Stone Age (c4000 BC in UK) and Bronze age. Stone tools continued to be used even into the 1600s - flintlock rifles and tinderboxes for lighting fires used flint. But these were tiny and if you've noticed something on the beach I'm guessing it's about hand size or bigger?