So…while i am not going to transcribe his piece on this…R. Herschel Schachter in Oct 2012 spoke about this issue. It is about on minute 40 in a shiur on YU Torah called, oddly enough, Tzizit. In short, he says a few possible reasons. (1) There is a medrash that seizes on the juxtaposition of “ki yikach isha” and “gidilim taaseh”. Those pesukim are next to each other…thus you dont wear a tallit until your married. (2) He said that there were certain places where only the Rav wore a tallit and for a “regular” person to wear a tallis would be “yuhara” — and he thinks perhaps that is where the ashkenazi minhag for singles not to wear a tallis comes from. He also points out that there is a minhag for single people not to wear a Kittel on YK. It could be that this is related. (Again — dont take my word for it you can listen for yourself).
I would take issue with one statement made here. [Not wearing a taliis for singles ” is not followed by most of the world.” Certainly in main stream ashkenazkic places (From Lakewood to YU) the minhag for singles not to wear a tallis. When my son made aliyah — he very much wanted to wear a tallis because even among the ashkenzim in the daati leumi world it is quite common for single men to wear a tallis. (I asked him not to. I probably should have just let him).