We got Cody neutered when he was 4 or 5 I think. I met Cody when he was 2 and then Jon and I started dating when Cody was 3. It took me a while to convince him he needed to have him neutered. All it took was Jon to hear the vet say it could help him live longer and we set up the apt.
I was really worried that it would change him, but it didn't and I am very thankful we had it done. With Boxers being prone to cancer I didn't want to risk anything.
I haven't read anything that states a cut off age. It didn't take Cody long to recover either. I thought it would since he was older, but it didn't.
Good Luck!