Yeah, I think it said he was bitten as a baby or a young child.

Edit:

Lupin paused and then said. “It was greyback who bit me.”

“What?” Said harry astonished. “When- when you were a kid you mean?”

“yes. My father offended him. I did not know for a very long time, the identity of the werewolf who attacked me; I Even felt pity for him, thinking that he had no control, knowing by then how it felt to transform. But greyback is not like that. At the full moon, he positions himself close to victims, ensuring that he is near enough to strike. He plans it all. And this is the man Voldmort is using to marshal the werewolves. I cannot pretend that my particular brand of reasoned argument is making much headway against greyback’s insistence that we werewolves deserve blood,that we ought to revenge ourselves on normal people.”