Maybe you should try to find the owner. It's the right thing to do. Post an ad in the paper, "found black dog please contact xxx-xxx-xxxx to determine if this is your dog." The found ads are generally free. Also post on craigslist. Take the dog to a vet and see if it is microchipped in attempts to locate the owner. I'd be highly upset if my dog got loose and someone decided they were going to keep it and not try to find me. Maybe there is a lost ad in the paper or on craigslist that is for the dog you found. Try to find the owner. It's selfish not to.
The dog is under 6 months old. Typically their testicles don't drop too early, and more than likely it's been losing teeth. A vet can best estimate age. You haven't given me enough info to attempt to estimate. A vet will estimate by the teeth. Not just "it's teething."