How can you tell the age of a kitten
If it has its eyes open then I would say about 2 weeks. If there are still slightly stuck in the corners 2-3 weeks. If it can poop in the litter trat and eat normal kitten food then 3-5 weeks. If its fat adn running around then 4-6 weeks. If it is really under nourashed then take it to the vet. If it gets diharea then it will only take a couple of days till it dies.
If you take it to the vet they will be able to give you an approximate age.
yeh take it to the vet thats the best option