This is interesting to me because my BP did not get a pink belly when in shed when he was small, but after he got bigger (and maybe hit snake puberty?) he started getting more and more pink before every shed. He is an albino so it shows really clearly and his belly now turns salmon pink before a shed but when he was small he didn’t turn pink at all.
This also led me to wonder if turning pink is age related but judging by the other comments here it isn’t. Maybe it varies from snake to snake, and maybe there are BPs who don’t turn pink at all out there.