Hi, i would say there is no set age as every dog/bitch is different and depends a lot on how much work they have done. Your knowledge of your terrier will be the best indicator of when the time for less has arrived.
That's what I was driving at..my friend still worked his dog at the age of 10...although I believe he (the dog, not the friend) was becoming senile which consequently led to aggression...but he still worked reasonably well; I think many will work until the day they cannot...but is that day assessed by us, or them?
Ironically, given the amount of experts on 'working' terr