I am looking for halachic origins. I am also curious about the Sephardic and Ashkenazim halachic justifications for their choice (when to wear it). I learned that Sephardic boys start wearing theirs at a young age, whereas Ashkenazim only start wearing theirs after their wedding day. Does this just boil down to personal preference? What do other Jewish communities do, besides Ashkenazim and Sephardim? What are your sources? When (point in time) did these differences in customs emerge? What caused this event to occur? Which group's custom is a violation? How do you know?
The page on Halachapedia here gives a clear rundown. I will try and link the sources accordingly (everyone feel free to add to any I can't find):
The Sephardic custom is to wear a Tallit Gadol from the age of chinuch in mitzvot (1). Chacham Ovadia adds that this is true even for a boy studying in an Ashkenazi yeshiva (2).
However, the minhag for most Ashkenazim seems to be not to wear one until one gets married unless one goes up to the Torah or leads prayers (3). Some acharonim(4) quote a midrash which learns from the juxtaposition of גדילים תעשה לך and כי יקח איש אשה (Devarim 22:12-13), that a man should not wear a Tallit until he gets married (5). Piskei Teshuvot 8:10 writes that this minhag spread to several countries in Europe including Lithuania and Poland, while in other Ashkenaz communities of Western Europe and in Hungary it did not catch on. Later acharonim (6) questioned these earlier acharonim and simply do not understand why someone who is not married would not fulfill this mitzvah from the Torah of wearing Tzitzit. Rav Y.D. Soloveitchik (quoted in Mipninei Harav pg. 22) says that in the absence of a minhag otherwise, the correct minhag is for an unmarried boy to wear a Tallit Gadol.
(1) Kaf Hachaim 8:12, Chacham Ovadia Yosef (Yechave Daat 4:2), Chacham Yitzchak Yosef (Yalkut Yosef 17:4 and Dinei Chinuch Katan pg. 29). Halacha Brurah 17:3 says that this age begins once the child can participate in the prayers in the shul. Ohr Litzion (Chelek 2, 2:7) says that the age is around 5.
(2) Rav Eliyahu Bakshi-Doron (Binyan Av Chelek 2, OC 7:1) agrees. see further Davening with a Minyan That Uses a Different Nusach - Tallit
(4) Tashbetz Katan 464, Sefer Hamanhig Chelek 2: Hilchot Nisuin pg. 539 - (need to pay to access), and Maharil Hilchot Ishut 10 - link needs payment to view but here is screenshot of the main point in the text:
(5) Tzitz Eliezer 20:8 notes that the Maharil only meant that before getting married one does not wear a Tallit Gadol, but certainly one should still wear a Tallit Katan (i.e. Tzitzit).