At what age do Female cats go into Heat?

 
At what age do Female cats go into Heat?
I know for certain that once Female’s cats are in heat, they aren’t out of it until they are Spayed.

Thank you, and serious answers only please!
answer:
My kittens went into heat recently, and they are both 6 months old. I’d say around 6-8 months old. It also depends on the cat…
cats normally reach sexual maturity at 5–10 months (females)

Source(s):

owner of numerous (fixed) cats
http://en.wikipedia.org/wiki/Cat#Reprodu…
Usually at about 6 months, but it can happen anytime between 4 months and 8 months of age.
As early as four months. Why haven't you bothered to get them spayed? It could have been done as early as EIGHT WEEKS.
AROUND A YEAR OLD.