In The Sorcerers Philosophers Stone, we learn about the stone and Nicholas Flamel who has lived for nigh on 600 years. Surely Voldemort would have heard of him as famous of an Alchemist as he was as well as this, Flamel had ties to Dumbledore making it even more likely that Voldemort would know of him.
Presumably, based on the stories of terror from his first reign he held more power back then. I'm unsure if Dumbledore was keeping the stone way back then (possible avenue to look into) but I would presume that if he weren't then Voldemort could have easily taken it from Nicholas.
What would stop a man so devoted to avoiding death to not seek one of most famous delayers of death?