I believe female cats can have their first heat cycle between the ages of 6-12 months and males can impregnate a female when they're as young as 5 months.

I've heard of cats being spayed or neutered at as young as 8 weeks. My newest addition, Mocha, was spayed when she was 4 months old and my other two were also spayed when they were young kittens.

I hope you plan on getting your cats fixed.