I first became involved with a non-orthodox congregation where it was customary for everyone over Bar Mitzvah age to wear a tallit gadol. However, I understand that it is an Ashkenazi minhag not to wear a tallit gadol before marriage. As an unmarried man who has been wearing a tallit gadol, can I change my custom and stop wearing a tallit gadol (until marriage)?
Actually, many Ashkenazim (mostly of German descent) do wear one before marriage. In fact, I've heard second-hand that Rav Yosef Dov Soloveitchik felt that the Minhag of not wearing one was wrong, so wrong that if there was any inkling of family Minhag to support wearing it, one should switch to wearing it if he didn't up to that point. I know one person who did just that.
Some Googling yields some insights into this, though nothing directly corroborating it.
Bottom line, those who don't are following the Maharil, and do(n't) so for family Minhag reasons, but if there's no family Minhag to go that route, it is seen (by some) as failing to fulfill a rabbinic obligation to wear one.