My boss recommends 6 months for female cats and 7 months for male cats. It's not a set in stone rule, he has sometimes neutered as young as 5 months if the cat starts acting like a tomcat (ie; fighting, spraying etc)

Dusty was already spayed when she showed up (RIP Dusty)

I had Jack neutered when he showed up at 7 months old

I had Pooky & Bear neutered at 5 1/2 months because they started fighting with each other and the other cats.

I had Cami spayed when she showed up at 1 year old.

I had Sassy & Sabrina spayed at 5 months because of having their intact brother in the house, I didn't want to take a chance on more kittens.

I had Josh neutered at 5 1/2 months because it worked out schedule wise for the surgery.