What age to cats come on there periods?

Kittens do not get 'periods', but can become sexually mature from 4 months (normal age 6-9 months). It is therefore important that you do not allow immature female cats out, and always get them spayed at an age the vet advises (usually 6 months). If they become pregnant when young it can be very dangerous, as they are not fully developed and the kittens may be too large to fit through the pelvis (this can be fatal).

Always ask your vet for advice!