Both of our cats are neutered. Our big cat, Louie, we adopted from Petco a while back, he was already fixed at that time so we didn't have to do anything. Our kitten we just recently got fixed, I think it was in November. He's only 7 or 8 months old right now. It was really funny though, when he came back from the vet it was like he didn't even notice and just ran hog wild all over the place as usual.
Since they're both boy cats there wasn't a pregnancy risk if we didn't neuter, but we didn't want the little one to get aggressive or territorial over the other cat. And we most definitely didn't want him to start marking his space haha.