Although a kitten from a humane society
will usually be neutered when you bring it home, I've never worked anywhere that wanted to do them before 6 months. My personal opinion is that they need some of the hormones they are developing to help them grow, etc., so they shouldn't be yanked out too soon!
I just had my puppy done at 7 months, hoping that she wouldn't go into heat before I got her in. It is routine surgery for puppies and kittens, but remember that there is a risk any time a surgical procedure is performed. If you want to have her declawed this would be a good time to do that,too - even though most places I worked would declaw them at 9 weeks. Better not to have them put under anesthesia any more than absolutely necessary.
Good luck! You'll have a better pet and be a better pet owner