Although the notion of your male kitten growing up and becoming an adult tomcat may sound far away and scary, it actually isn't. Cats grow up fast -- yikes! Unneutered male kittens often act very differently than their fixed counterparts, from territorial urine marking to aggressive fighting behaviors.
Before the general age of feline sexual maturity -- roughly 5 to 6 months -- fixed and unfixed kittens display very similar behavior patterns. After all, the hormones aren't pumping through the wee ones yet. Typical kitten behavior involves a lot of rambunctious play, licking, learning to interact with peers and beginning grooming activities. You probably won't notice a discernible difference -- if any -- in both types of kittens before this age, even though many are fixed before they even are old enough to possess sexual behaviors.
Kittens aged 5 months and up may begin territorial urine spraying if they are unneutered. This behavior has a dual purpose -- to give off scent in order to attract nearby "in heat" females for mating, and to show the competition who's boss. However, neutered male kittens may continue this behavior out of habit if they were already accustomed to spraying, although this is quite rare.
Unneutered male kittens are usually significantly more aggressive than their fixed counterparts. Fights are a common, everyday occurrence for tomcats. Competition and testosterone drive the tomcat world, often to the point of danger. Older unneutered male kittens, if given outdoor access, may frequently show up at the end of the day with painful-looking abscesses, open sores and bite-induced infection -- ouch.
Unneutered male kittens also have the tendency to want to wander off more. Once a kitten is old enough to mate, then his thoughts will center primarily around doing so. Your hormonally-driven pet may be restless and antsy all day, pawing and clawing at all of your doors and windows so he can get outside and hang with the lady cats!
It is not uncommon for hospitals and clinics to neuter male kittens as young as 2 months. Speak with your veterinarian about what time frame is most suitable for your male cat. With early neutering, you may prevent the emergence of unpleasant mating behaviors such as urine marking and aggression. Apart from keeping your precious pet more mild-mannered and calm, you also will help keep troubling cat overpopulation in your area to a minimum -- a definite bonus.