if you keep and work a good dog and it does its job for you there is no reason to part with it. except in extremely rare situations.
i dont do an awfull lot of terrier work these days but the best i got out of my terriers was from around four years onward. finding they will only get better and better with work. without doubt the best lads ive met always have one or two old ens about the place.
if they go that extra yard for you , you should go that extra yard for them this iis just my take on things.