I believe that the Minhag is based on a Gemara in Kiddushin (around daf 30) which mentions that it is yuhara for a non-married person to wear a beged over his head. The interpretation is that it’s talking about a Talit Gadol. That would not seem to have an age limit.
Apparently, Sephardim either learn the Gemara differently – saying that the beged mentioned is not a Talit Gadol. Or that if everyone does it, then it no longer becomes a chashuv way of acting.
From what I’ve seen, young yekkes wear a TG with a hat, so that they don’t wear the TG over their heads.