Is there any way to determine a cat's age?

If he definitely has some baby teeth, he is under 6 months. If they are ALL baby teeth, then he is up to about 14-17 weeks old. At that age, he starts to lose baby teeth, and the adult teeth start growing in. By 6 months the adult teeth are all in. This is more accurate with dogs, but works well with cats, too. A vet can give you a closer opinion. Also can give you the vaccinations he needs -- he's very, very susceptible to certain deadly illnesses right now at such a young age, so the vaccines are very important. Size also can give you a good idea of the kitten's age, but it is hard for me to describe here. If he is around 2 months, he looks very small and baby-ish. By 3.5 to 4 months, he is just starting to look bigger and get just a little gangly (longish legs and body may be slightly less proportionate). Around 6 months he looks teenager-ish, and by 8 months he will look adult, but he will still be a kitten.

Your vet can age the cat, likely based on his teeth. Particularly when they are young. It is harder to age an old cat by their teeth, unless they are very old. Sorry, I don't know how to age a cat though. My guess is that he is more like 5 - 6 mos old, not 3 mos like you thought. My vet told me that they would start to breed as young as 5 mos, therefore also spray to mark territory. I don't think they will breed/spray younger than that. It is better to get a male kitten fixed as young as your vet will agree to do it, to PREVENT spraying before it starts. Once it becomes a habit, it is very hard to stop. But sometimes, fixing them, stops the urge to mate, which also stops the urge to mark territory, because they are related activities.