How young is too young?

I have cats who got pregnant at the very young age but she survive only for a month. I think their body is not yet metabolize or matured enough to handle being a mother. All of her kids were still alive today minus the mother dear. I am their mother now.

I am sure she will be fine. My female cat went into heat at 5 months. I kept her indoors away from the boys, and got her fixed right away. The vet said she was still in heat, but was not pregnant. My mother once had her female cat fixed, and it turned out it was pregnant. The cat basically had an abortion. But I am sure if she is capable of going into heat and getting pregnant, then she is capable of carrying the kittens full term.

I want to say that it is 4 or 5 months. I believe that cats don't go into heat until they are capable of carring a litter.