i think that the rule of thumb is that its acceptable up to the age of 5, after that they really do know the difference between a boy and a girl and would notice if someone was changing and they are old enough to follow basic directions and go in and change by themselves without getting into trouble. after 5, the parents of the opposite gender can just stand outside so the child can yell if they need help and no one could sneak them out because the parent would be standing right at the door, also they can follow the basic rule to kick and scream if anyone tries anything. i dont assume that every man over the age of 20 is a pedophile, if i was feeling really insecure then i would just look for a man with kids going in and ask him to keep an eye on the child. after 5, its just creepy and inappropriate. if hes too old to change in front of others, hes too old to be in the girls room.