How old is masturbation and its history?

There was a discussion in a recent thread about how old masturbation is and its history. It intrigued me when someone said it was a relatively new thing.

Personally I think masturbation has existed since ancient time in the thousands of years. It has been in art and in writings. Prostitution has existed for more than a century so why not masturbation?

It has been referred to as "onanism" in some religious texts to which indicate to it being quite old.

What are your thoughts on the subject?