It is not a halacha and it is not followed by most of the world.
Yekkes historically wear the tallet gadol from the time of bar mitzvah, which the Chofetz Chaim believed was correct.
This is purely a minhag, one without sufficient grounding, based on the different rayas I’ve seen (such as the idea that a tallis is equated with ohr hamakif which indicates simcha, and an individual unmarried has no simcha and therefore isn’t on the madrega of wearing a tallis.
If anyone has a sufficient reason other than minhag brecht a din (sic), let’s see it.