it all depends mate let your puppy be a puppy for as long as possible, i made this mistake once and boy did i leanr by it, by entering a pup too early, they'll either take a nip and shit themselves and not want to work to ground or in my case go totally the other way, your dog'll go fox daft smashing into its quarry and coming out of a dig needing a good 2 weeks rest to heal which is the last thing you need as the season only lasts so long, best of luck with your terrier mate, theres no rush to enter your dog let it enter on its own accord.
hope i've helped
cheers ryan