I dont think there are separate rules for adults and juniors - so in the BD rule book anyone can wear long boots or gaiters and short boots, entirely up to the rider.
Just had a quick look at Phoebe Peters' website (she is 14) and she wears long boots so cant see why you would have a problem wearing long boots. I think the only reason short boots are popular on kids are because they are cheaper for the parents to keep replacing each time they grow!