I was trying to find the information about youngest player with ATP World Tour title.
I found on Wikipedia that Michael Chang is the youngest player to win Grand Slam title. Wikipedia also has list of oldest tournament winners (current revision). But I did not find information about who was the youngest tournament winner.
If you mean any ATP tournament (and not the overall Finals), Aaron Krickstein won an ATP World Tour tournament in Tel Aviv in 1983 at the age of 16 years 2 months, the youngest to do so according to his wikipedia page (which cites a book with that information). I don't see any other source that disagrees, and he's still listed in Wikipedia as the youngest top 10 in the ATP rankings, so that seems likely.
