If you want to be completely pedantic about it, a master craftsmen is simply a person who has created, and successfully defended, a masterpiece (basically the medieval craft/trade-guild equivalent to a, well, Master's thesis.) This happened pretty early in life (you didn't enter a guild in full good standing until you became a master), so an arbitrarily-selected "master" wouldn't be nearly as experienced/talented as the few that come to mind when you think of the term.