Not wearing a talis after bar mitzvah! sure we come up with very weak justification such as a “hekish” of ki yikach ish and gedilim taseh
(Though to be sure, there is no halacha you HAVE to wear a talis, so it also might not be the best example)
This ‘weak justification’ comes from Rishonim, so it doesn’t matter if you understand it or not. Again, this isn’t Minhag v. Halachah, it is Halachah v. Your Question.
My minhag is to wear a Tallis from Bar Mitzvah [and a little before], and I too never understand the rationale behind not wearing. The Hekesh, if anything, would apply to Tzitzis as well. <Disclaimer: I haven’t researched the inyan properly>. But the other Minhag has a very reliable base in Halachah, not opposed to Halachah.