There are so many controversial conversations as to when a kitten must be sterilised. With little boys, you can only sterilise them when you see their little testicles. If not, then what will the vet remove? The testicles should be appearing from 9 weeks old. From this time onwards is a good time to sterilise.
For little girls, there are also a lot of controversy. Some say 8 weeks, some say 6 months. But due to the pregnancy cycle, a female kitten should be sterilised when she is 1kg or 12 weeks old. Kittens an fall pregnant at the tender age of 4 months!