As Nicholas Flamel was 665 years old by the time of HP&TPS, It's likely that the Philosopher's Stone had been in existence for at least 500 years.
Considering the extraordinary lengths to which it was protected during the events of Harry's first year, it seemed to me that the protection was a relatively new occurrence, something that had never been done before (harboring it inside a school at the very least doesn't seem to be something to make a habit of).
However, it's difficult to believe that in the 500 years that it had already existed, no one other than Voldemort had thought to attempt to steal it to gain unnaturally long life.
Is there any evidence that anyone had attempted to steal it before this? Was it as heavily guarded before this point?