I think 13 weeks is fine, I had my cat done at 12 weeks.

I think there may be some "old school" beliefs about sexual maturity and health still in play, but I've not heard any evidence that waiting leads to healthier cats.

When I was growing up, there was a belief that you should not alter a male cat until an older age to prevent bladder problems later. We waited until 9 months to neuter our orange tabby because that thought process was still in play back in the 90s in some places. We scheduled his neuter the day after he began acting... like a cat with male hormones. We could not have waited that long with our recent addition, he was already acting like a "man cat" by 5 months.

Our current vet had recommended 5 months for all kittens, but said that 4 months was fine if the kitty was acting out before then.

I'd get go ahead and get them done sooner than later. This recent one we had done was the youngest, but it also seemed like the easiest too.

My Ragdolls were done at about 10 weeks old. No issues. Ragdolls "run big", but I'd say most 13 week old cats are as big as a 10 week old Ragdoll.

You probably don't want to hear this, but if you really don't want to endanger them you shouldn't let them be outdoor cats. Outdoor cats live far less long than house cats. They get hit by cars, eaten by coyotes, attacked by all manner of creatures (dogs, other cats, nasty people), ingest poison, etc.

At the shelter, 2lb and up is a go. Rescues want to get kittens adopted as soon as possible. Never had problems with the cats done at that age

My own that I decided to keep and it was up to me when to get it done, I waited till 4 months.

I had one litter I was fostering - they had ringworm and as a result, stayed in my foster care longer. Around 5-6 months, the litter mates started doing *it* and I had to schedule boys for neuter ASAP. We did not want them right away when they were younger because the ringworm was really bad, but when they started trying to mate, it was clear we could not wait any longer.
I cannot imagine anyone wanting to wait till 9 months with boys.

I took in an abandoned male cat, who may or may not have been small for his age because of his circumstances. I figured him for about four months tops......and was going to get him neutered as soon as he got healthier and a bit older, but he beat me to it and my household had the one and only kitten born in it in twenty five years.

Check your local shelter to see if they have a reduced fee spay/neuter program.

Our local rescue spays and neuters once they weigh 2 lbs. In order to be sure they new owners don't "forget" to take care of it, kittens will not be homed until they are done.
The group also takes care of some feral colonies, and kittens that young are placed in homes.
It certainly has reduced the number of dumped kittens.

Thanks for the encouragement. They are scheduled to be altered in three weeks.