First of all, commenting on the last comment, I think the war of Harry Potter vs. Twilight is STUPID! There is nothing similar about the series! Plus, I don't have a problem with Twilight.
Anyways, by the time Dumbledore met Tom, it was, technically, already too late. Tom had already developed a habit of hurting people & collecting trophies. Dumbledore tried to show him that what he was doing was wrong, but it had no affect. Plus, Tom already had a liking for acting solo. He didn't want companionship, so Dumbledore adopting him wouldn't've worked.
Also, Tom acted like a perfectly normal kid in front of all the other teachers, and was very cautious around Dumbledore. So, while Dumbledore DID keep an eye on him, he also considered the possibility that Tom felt sorry for what he had done, and was ready to 'turn over a fresh leaf'.
He would've become Voldemort, regardless. He hated his name.
Hope that helps.