You can determine the age of a rock by looking how rough the rocks are... so lets say you fine a rock that has many points, sides, and rough edges which means the rock is actually pretty young so it was recently created and most likely a sedimentary rock. But if you find a rock that has smooth sides and no rough edges which would mean the rock is pretty old rocks could go up to thousands and even million's of years old and if a rock is that smooth it's certainly old. This is actually called: "rock identification" which is really easy to learn.
Hope this helps!