Just because sanskrit isn't that old it does not negate the dating of the purana. Many ancient texts are being found to have been written an more ancient languages than sanskrit then translated into sanskrit at a later date. Sanskrit is not the be all and end all of ancient language. In fact what is known as classical sanskrit is not even the original sanskrit. Ancient sanskrit is the forrunner. Classical sanskrit is a poor re-write of ancient sanskrit and distorts the meaning and meter of ancient sanskrit. The ancient texts that were written in ancient sanskrit then transcribed into classical sanskrit are soreley mangled. You might be surprised to know that Tamil is as ancient or more ancient than sanskrit and thousands of verses are written in ancient tamil.