just my own opinion but to me at 5/6 mth old and allowed to enter is too young, im talking adult quarry, a fox can bolt whilst netting up before a terrier is put to ground, a quick bolt doesnt mean its worked it, i do let my pups have a look around 8/9 mth at the end of digs but dont allow them very close contact, then ile hold them back till around 13/14mth mark if i think there ready, then ile let them have a easy doo, then if im happy with them then its there chance after that to do the job i ask from them, (find and stay) to them that start them young ATB