The mitzvah of tzitzit is discussed in the third section of the Shema which is recited during the morning prayers. Therefore, during the morning prayers it is customary to don a tallit gadol ("big tallit") -- a prayer shawl. Wearing a tallit is the ideal way to observe the mitzvah, for only in a tallit is the individual enwrapped in the garment.
In most Ashkenazi communities, men begin to wear the tallit only after marriage (and continue to do so even if they are widowed or divorced). According to Sephardic tradition, boys begin wearing a tallit at the age of bar mitzvah or even earlier.
In most communities, a person who is called up to the Torah for an aliyah--even if he is a bachelor or isn't praying at the moment--dons a tallit out of respect for the congregation. Similarly, the chazzan always wears a tallit, even if he isn't married, and even during the afternoon and evening services, when he is the only one who wears the tallit. On the other hand, many, including Chabad, do not follow this tradition.
