It's best to wait until they are a year old. I for one do not believe in
breeding & have my pets spayed & nuetered. Why do you ask?

Normally cats start their heat cycles around the age of 6 months.

Depends.  If you are asking because you have a wonderful
cat that you just HAVE to breed than wait until she is at
least one year old.  They can get pregnant sooner but it's
not good for the cat or the kittens.

If your asking because you want to know how early
to alter your cat to help stem the population problem
than do so before she comes into her first heat (or keep
her locked up).  Some say 6 months and I think that
is as early as they do it in the UK.  In the US you can
do it as early as 2 lbs (appx 10 weeks) so I would do
it by 4 months just to be on the safe side.  Some cats
do come into heat early.

AG