Where does the observance of only married men wearing tallit gadol come from? Is it minhag or halachah?

It is the custom in most Ashkenazi communities. In Sep[hardic communities and in German communities unmarried men wear a tallit gadol. The source for the distinction is not clear. Tosophot in Brachot 14b says that wearing a tallit during tefilah is not manditory. Some say the married man should wear the tallit because of its effectiveness in keeping one's mind off of forbidden liasons. It is not clear why this would only apply to married men.