My first bitch was entered at 18 months, i knew she was well bred and had the pedigree to take foxes, but you never know until the job is completed,, even if the dog was to attack the fox and feel the sharp end would the dog continue,,, thankfully my bitch DID! from then on her pups at the same age would of been entered with their mother to show them the way,,, i would do this on the lamp and at the dig.. And most of all i would judge the dog myself on how it handled the fox,, not let some gobshite tell me otherwise.