First heat can vary greatly dog to dog. The youngest is about six months of age though sometimes a female will come into season younger. First heat can start as late as 12 or even 14 months of age or later in rare cases. Again, it can vary dog to dog. Spaying at six months is recommended before a female begins to enter sexual maturity. Some vets will spay younger and spaying of puppies as young as eight weeks old who are up for adoption in shelters is becoming more popular.