Determining a Cat's Age?

As some of you may know, I recently adopted a young female cat whom we named Emma. She was found huddled behind a dumpster in an alley during a snowstorm--so no one's sure exactly where she came from, how old she is, if she has previous owners, etc. There are only a few facts that we have to piece together to try to figure out her age (so we can assign her a birthday!).

First fact: When she was found, the vet who examined her was pretty sure she had recently had a litter of kittens(she has since been spayed--the vet who spayed her confirmed that she had at least one litter--but probably only one due to the condition of her uterus--yes, kinda gross, but there you have it.)
. The same vet thinks she's under a year old, but can't be sure of her exact age. So, there are a couple of questions I have that might help pinpoint her age a bit more:
How old does a femal cat have to be to have her first estrus cycle?
Exactly how long is the gestation period?

Any help would be appreciated!

around 6 months is her first heat although my cats was 9 months

gestation is 57-70 days 65 being the norm.

she can get pregnant again shortly after kits are born.

how much does she weigh and what breed is she? is she short or long hair?

Female cats can be pregnant with seperate litters from different (or the same) male at the same time. So a cat can have a litter and then a week (or2 or 3) later have another. We have had a couple at the shelter do that. I never knew it was possible.

You can tell the age by the condition of the teeth but it is hard because there are so many factors to consider. It will still be a very rough estimate and you can be off by a year or even more. Good luck.

She's about 5 pounds, very very tiny bone structure, DSH Tuxedo with some Persian or other "smushy-faced" breed involved.

a small long haired cat (like mine) may not come into heat until 9 months old or older. i *think* the majority of females come into heat when they reach 6months or 5pounds, which ever is soonest.

A stray appeared several yrs ago and she was small and seemed old. She just didn't move around very much-happy to eat and sleep She didn't stay long but slept alot in the family room. One day she just wandered away-happy I hope. I examined her teeth and they had long grooves on them . My Sheba who was 9 when she was killed-her teeth were starting to have these grooves on them also though not yet as deep. Is she missing any teeth? Or signs of gum disease?

She's not missing any teeth, and is as playful as a kitten. I suspect she's somewhere between 6 months and a year...was just thinking perhaps we'd be able to pinpoint it a little bit more in order to select her birthday

A female cat can go into heat as early as four months. An early pregnancy could explain her small stature, as it would have stunted her growth. (I've seen it before with some of the young pregnant cats at the shelter, and also with young female dogs who were kept at puppy mills.)

Thanks so much everyone for the tidbits of knowledge!
We're going to say she's 10 months old, which is what the vet-guesstimated.
I wonder if she'll grow a little now that she's not on the street and pregnant?

Well, I'll keep you posted!

It's possible. A kitten can grow for a little more than a year.

Make sure to keep her on kitten food for at least another 2 months, just so she has the nutrients to be able to keep growing!

There are also cat breeds like ragdolls and ragamuffins who supposedly grow for the first 4 years of their lives and become pretty good sized cats. At least thats what I read about them when I was trying to figure out if a kitten I found 1.5 years ago was one or not. She has grown sooo much and is the biggest, longest cat I have ever seen and still growing.