Where I work at they won't spay/neuter any animal until 5-6 months old. The theory is (at least with cats anyways) is that they have a naturally higher crystal content in their urine anyways and having the surgery done when they are a few months older allows the urethra (the part the urine passes through) to grow a bit more which should decrease incidents of being blocked up when they are older. In male dogs waiting seems to allow them to look more masculine than younger neutering. I honestly can't say for sure on the female dogs young vs older spaying as I've never actually asked before, I just know it's better to get them done BEFORE their first heat cycle which is around 6 months old.

For my own cats:

Jack was neutered at 8 months old when he got his second set of shots and has only had one minor UTI several years ago which caused no blockage and was treated with Amoxi-drops for a week. He has not had that problem since then and he's now almost 8 years old.

Pooky & Bear were neutered at 5 1/2 months old because they had started acting like tom cats. Neither one has had any kind of urinary problem and they are now 5 1/2 years old.

Cami was spayed when she was 1 year old (when she showed up) and has not had any urinary problems. She is 3 years old.

Sabrina, Sassy & Josh were spayed/neutered at 6 months old and have not had any problems but they are only just now 1 year old.