From the Sacramento Area Animal Coalition. At what age do female cats first go in to heat? Female cats typically reach sexual maturity when they are about 4.5 – 7.0 lb (2.3-3.2 kg), or between five and nine months of age. However, it is possible for female kittens as young as four months to go into heat and become pregnant! In addition, domestic shorthair and longhair cats may reach sexual maturity earlier than purebred cats, and free-roaming cats may become sexually mature sooner than cats kept indoors.