I don't know about prime but I don't think a terrier is fully mature 'till it's three. Obviously they're all different (ie. bitches often mature earlier than dogs) but as a rule I think they mature at around three.
As for prime, they're all different. I dug to a 9 year old bitch 3 times last week, twice one day and once a few days later. The same bitches sister was retired after 3 seasons.
I've dug to 7, 8, 9 and 10 year old terriers that could still succeed where other fellows prides and joys wouldn't at 3 and 4.
There's not to many rules in this game regarding the working life of a terri