The Stone Age is thought to have started 2.5 million years ago (that’s a seriously long time ago!!)
As carol says we know things from what has been found. For example animal bones have been found with marks caused by stone age tools. The bones were dated back to that period so we know that people were hunting then.
Also archeologists have found tools in places like Ethiopia which were made from flint.
I think it must be really cool to be an archeologist although hard work digging stuff up!