Reference:
Our cat came into season for the first time when she was just 5 months old. We had already decided to let her have one litter of kitties before having her speyed, and the vet gave her a contraceptive injection which covered her for about 6 months until she was more mature.
My kitten is at least twice that age and has shown no sign of being in season. Katty, don't worry about unwanted kittens. I would not let her have a litter if I didn't have homes for them.