Is the cat old enough to eat solid food/drink liquids on it's own yet? If so, provide it with water, warm milk, and kitten food (which is a soft, smaller version of cat food). If the kitten is too young to drink milk out of a saucer, you're going to have to do the honours. The easiest thing to use is an
oral syringe, which is commonly used for giving liquid medicine to babies, and is available from any drug store (Walgreen's would always give us one free with a prescription for my neice/nephew when they were babies).
As far as an age estimate goes, are it's eyes and ears both open? Can it run and climb yet? The youngest kittens can usually be seperated from their mothers is at about 5 or 6 weeks old. If the kitten can eat food on it's own, I'd say it's good for now.
As Jellyfish said, take it to a vet/shelter to get it checked up, and also check with local animal shelters to see if anyone's looking for this particular kitten. You could also try putting a Found ad in the Lost & Found section of your local paper's classifieds.
Make sure you provide the kitten a nice warm place to sleep. Safe toys to play with, and good food to eat. And a lot of love, kitties like love. Also, this kitten's at the age where you need to litter train it. So get a box and get going.