I think it's more of a weight limit actually. Like weight/size. At least I know with two coaches I know... around the kid being I think like 90lbs or 14 they are taken away as an... almost ritual, of becoming a rider of a certain level. Happened to me (and I was scared shitless lol! odd child)

They do make a safety/release stirrup without elastic, it's bent oddly and I can't remember the names of them but yeah those can be used by an adult rider.

Too bad you don't know what the sire was before grey. That would help.