Most A & B Nationals when we did them conformed to that as well. Sorry to say I do not think it was a ruling as at least two regulars of those we did, (Over several years), did involve a considerable amount of "hiking". I'm afraid these days many parents would not countenance having Kids under teenage years out of their sight. Not because they didn't trust anybody but because if anything did they need to be around as to avoid the "Heath and Safety" musketeers making assumptions they shouldn't.