The guy who made the Philosopher's Stone died of old age when he and Dumbledore agreed to destroy the thing. It's kind of funny imagining a crazy scientist walking up to the Dean of Harvard and saying, "Hey, look, I invented immortality, should we destroy this or something?" but hey.
Voldemort pretty much stayed alive through consumption of unicorn blood. The horcruxes only prevented early death - Death from aging was still a possibility. After the first book, though, how he didn't die from aging was kind of hand waved. The reason he was so obsessed about the Deathly Hallows was because they would prevent death from aging, as well.
I have no clue why I remember all this.