A. The Talmud Tractate Kiddushin implies that unmarried men do not cover their heads with a cloth, and many understand this to refer to the wearing of a Tallis, as opposed to a Tallit-Kattan which is only worn on the chest/torso. Sephardic Jews, those of German descent and many modern congregations wear a Tallit from the age of 13 and some even earlier.
This question has nothing to do with the Mitzvah of wearing Tzitzit garments which are worn by young boys as well, and relates only to the full Tallit. See this question about the difference between Tallis and Tzitzit.