Young female cats can first come into heat as young as 5 months of age, but usually don't until between 6 and 9 months. Some may not cycle until one year of age. To avoid pregnancy, cancer of the reproductive system, and the potential for infections of the uterus later in life, your pet should be spayed by 6 months of age. This holds true for indoor cats as well. During a heat cycle, even timid, young, indoor cats may do their best to get outside to reach males. It only takes one quick escape outdoors for your feline friend to become pregnant.