We are never done learning in this trade. It really depends on the person, types of work he/she was exposed to and more importantly their master plumber responsible for their training. I have met a 4th year apprentice that said a shower valve was made of hard copper. Another one sold himself as having enough hours to be able to take the journeyman test but did not know how to install a frost free hosebib. Both came from a company that does apartment buildings and never really teach anything but just tells them what to do and they do it like mindless drones. We will always have more to learn but what's important is how we deal with it when we encounter new challenges.I require 10 years experience minimum before I hire a plumber.
Even at 15, 20 years I still had a lot to learn.