20.
Remus remained friends with Lily for the rest of her life; his friendship with Severus lasted only until fifth year when a certain prank turned Severus against him. James and Sirius would always remain his closest friends throughout school and even afterwards. For the rest of second year, none of them spoke to Peter and they guarded Remus most carefully; it wasn't until the middle of their fourth year that Peter was welcomed back as he had changed over the summer and had apologized most profusely to everyone, but especially Remus. Though James and Sirius were never "friends" with Severus, they stopped bothering him as much, even if it was only for a short time.
Though Peter was "friends" again with the Marauders, he wasn't fully trusted or accepted until the end of their sixth year…even then Sirius was the one who remained skeptic and cold at times towards him. The four of them were always together, and most would say that they were as close as siblings…if it were not for one thing that happened on a certain Halloween night and the following day, however even then no one suspected what had really happened.
Peter's thoughts.
They had contacted him; they had seen, approved and congratulated him for what he had done to the filthy half-breed. It had to remain a secret, especially from his "friends"; even if they weren't his friends anymore. They had betrayed him and gone over to the freak, they would rather be friends with a monster than with him. Well, he could pretend to be his friend too and then when he was ready; they would all pay.
For the rest of their time at school he was polite and friendly, deceiving all of them every day, and making them believe that he had changed enough to be at least civil to the monster. He even managed to joke around, and invite it over during the summers. He had succeeded and he had fooled them all.
As soon as they left school, he got to work. Joining those who he had been in contact with for so many years, but didn't let anyone know; becoming a spy for the Dark Lord and telling him the secrets he learned as soon as he got away from the meetings with the others. Soon he began to let things slip to the others, things that suggested that the freak wasn't trustworthy, that it was a spy for the Death Eaters. They believed him, or were starting to; all he needed was a little more time. For now the monster wasn't trustworthy anymore and he was ready to put the next part of his revenge into action.
He had absolutely no qualms about telling the Dark Lord where James and Lily lived, why would he, they would rather be friends with the half-breed too, so they should pay. The Dark Lord had been proud of him and for this information, he himself was gleeful when they were dead and he was ready for when Black came for him. He let him think that he was cornered, made sure that the ministry was on their way when he shouted that Black had betrayed James and Lily, smiling as Black raised his wand and he cut off his finger before transforming into a rat. Hunched in the sewers he listened as Black was led away laughing, one of his two regrets was that now he couldn't finish the freak, for it had gone into hiding. And now with the Dark Lord, dead he had nowhere to go, yet he had done it, he had made them all pay and revenge was wonderful. The best part? Even now no one—well other than Black, but he was in prison and no one would listen to him—suspected him and he had died a "hero's" death.