Well, there is a perfect quote from Doctor Who to describe the Marauders. All four of them:
Every time you see them happy you remember how sad they’re going to be. And it breaks your heart. Because what’s the point in them being happy now if they’re going to be sad later. The answer is, of course, because they a re going to be sad later.
Peter was a Marauder. He spent seven wonderful years with James, Sirius, and Remus. He might not have been as close as the other three nor as brave nor as strong but he was still a Marauder. He was right next to them when they participated in their endless pranks. He still took the Animagus potion to help Remus. I mean, think about that! Peter has this stereotype of being a coward and he still ran around with a werewolf once a month! He laughed with them, he cried with them, he fought with them, he pranked with them, he loved them. He was a Marauder, through and through. And nothing will convince me otherwise.
The years between their graduation and October 31st 1981 are unknown to us. I’m sure that he was still a Marauder for at least a few months prior to graduation. However, we also know that he eventually became a double-agent for Voldemort. The day that he decided to become the double-agent was the day that he stopped being a Marauder. That was a day that his mischief ended. But he was a Marauder every day before that.