Tony: Where the hell are we?
Percy: Geographically speaking, in the Northern Hemisphere. Socially, on the margins. And narratively, with some way to go.
Dr. Parnassus: Percy, what would I do without you?
Percy: Get a midget.
Tony: Don't shoot the messenger!
Mr. Nick: You're probably not a betting man are you?
Tony: Can you put a price on your dreams?
Dr. Parnassus: You can't stop a story being told.
Mr. Nick: That's a weak hypothesis.
Tony: The Imaginarium does not cost a thing. We're not here for money.
Tony: Don't believe everything you read. Especially The Mirror.
Tony: Nothing is permanent, not even death.
Answer: Percy's backstory is never touched upon, but he certainly appears to have been with Parnassus for a substantial period of time, longer than his visual age would suggest. Possibly he himself has done his own deal with the Devil, or some other power, to extend his life, but, if so, the film never elaborates on it.
Tailkinker