My vet wouldn't neuter Rudy till he was 6 months old. And believe me I was happy to see that day come.
He was humping everything. That was 6 months ago and after that I haven't seen any personality change in him. Every now & then he will still hump something, but he's still the same wild Rudy as before.
Some people even ask us if we've had him neutered, cause they say it usually slows them down, Not My Rudy. Hee! Hee!