How old should my cat be to be spayed?

It sure is! Most cats now are altered based on weight, not age. Males can be done at 2 pounds (about 8 weeks) and females at 2 1/2 pounds (about 10 weeks). It's called a pediatric alter, and it's been done in the US for over 20 years. She really has more health risks if you wait, than if you have her spayed now. As long as she's healthy, call the vet today or Monday and make her appointment. The sooner she's spayed, the sooner you get it over with and then you don't have to worry about her coming into heat and getting outside. At her age - 5 months - she's going to come in to heat any time now.