Wow...a whole 2.5 hours went by without an answer. Patience is a virtue....
If it's crunching dry food, I'm assuming it has teeth which would mean it's more than 3-4 weeks old. If you google "kitten development" you will find plenty of pages that describe their progress week by week, which should help you figure out age. I would offer canned food, mix in some kitten formula to ensure it's getting plenty of calories.
You mention that your resident cats aren't too happy about this little one. Hopefully you have the kitten isolated and have not introduced him to your cats. You have no idea what diseases it can be carrying...anything from worms to FeLV or FIV. If you decide to keep it, a vet visit is needed asap for bloodwork, deworming and possibly vaccines (depending on how old he is). I would keep him isolated for at least 10 days to make sure no health issues are brewing. If you're not going to keep him, I wouldn't introduce the resident cats at all.
If you post a picture we can probably hazard a guess at age (and we'd get our kitten fix).
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Holly, Misty & their friend, Jake, the dog.
Onyx, Callie May, Maggie & Kobi forever in my heart.
Yes, I know I need a new signature pic...someday...