your cat is about 4 months old after what i can see but this is an estimate so it can be two weeks younger or older.
i base this on body shape and on how developed the limbs are and the fact that the muscles are not yet fully developed.
so you have a little teen cat in your house,remember to get it to the vet for a health check and vaccsination and to get it vaccsinated and spayed/neutered.
you need to be sure it do not have an owner before you addopt it,some child might have lost their kitten so this is something you need to find out before you do anything.