DY: But normally, you don’t have to check, so as far as we are concerned, he is “Bar Mitzva”, due to the Chazaka, and we have no chiyuv to check.
Here, as with other dinei D’Oraysah (such as Laining Zachor), the individual must be checked.
Yes, I was just nitpicking on the distinction between a “bar mitzvah” and a “bona fide godol”. I would have distinguished between a “13 year old” and a “bona fide godol”.
I fail to see the need to wait beyond the thirteenth birthday (assuming the hairs are present). It can easily be determined via medical exam if the hairs are present (if the boy himself is not believed).
AS per Shulchan Aruch (167 – 3) we do indeed check (but he is not believed because we would first need to determine if he’s a gadol before we give him credibility).