A cat can be spayed/neutered at any age - and if a female is in heat a vet can spay her at that time as well, but they charge more.
For a male, one thing to watch as I just went through this - this is advice that the animal hospital we went to informed us of - a neutered male is at a higher risk for Urinary tract infections. So it is good to make yourself aware of the symptoms - (and they come on really, really fast!) If you can, switch the cat to a Urinary Tract Health diet (lots of brands make them now - right now I am using Hill's Science Diet which is expensive and by prescription only, but less expensive brands are available from pet food stores.) Make sure your cat has PLENTY of fresh water - my Siamese who was put down this weekend - was fussy about his water - if it was more than 4 hours old, he wouldn't drink it. So I paid the money ($30) for a circulating water dish. I bought it at Walmart. Every 15 minutes or so, it circulates and filters the water. The cats drink much more actively from that than their glass dish.