When it was mentioned that pre-marriage yekkes don’t wear tallisim over their heads on this thread, it was referring to throughout the davening. Of course it’s still the custom to wrap it around the head right after the brachah is said, so there is no problem with the Arizal quote that HaLeivi mentioned.
Also, from my experience, in Sephardi communities AND in minhag Ashkenaz the minhag is to wear a tallis gadol well before bar mitzvah. Is there anyone outside of the hederodoxy and Modern Orthodoxy that start exactly at bar mitzvah age? I see that a lot of people on this thread can argue the logic of starting at that point, but that’s not how minhagim are decided. The custom to wear it from age ~3 clearly goes way back, as does the custom to wait until marriage. What’s the basis to start at the bar mitzvah?