The one I started at 10 month was marking to ground from 8 month old, a real early starter. He was a strong terrier also, so I thought he's wanting to go and he's physically strong enough to handle what's in there so let him go. He was in and out on the first time, just a bit unsure of himself but we got the result with him in the end, his second time to ground he seen the job through to the end without showing, a stiff 4 foot dig to a fox but he handled it, he ended up being a decent digging dog but having faults aswell, and died to ground at 3 1/2 year old.