All the more reason for there to be some kind of adequate skill benchmark system. A blacksmith currently does need to be of a certain level to take an apprentice; typically a craftsman can't take an apprentice until he has acheived mastery in his craft. Journeymen do most of the real work most people see, but generally guild rules won't let you apprentice someone to yourself unless you're a master yourself.
About the only real skill benchmarking system I know of is in Mutants and Masterminds. There, 1-4 ranks is basic, 5-8 is a professional, 9-12 is an expert or master, and 13-15 is a true 'wizard'. Someone with 16+ is a legend. So, I'd say someone with 10 ranks is qualified to take an apprentice. That means 7th level Expert for the Master Blacksmith in the Big Town, since that's the earliest he can get 10 ranks - Skill focus and all that mean nothing since they just add to the bonus, not the number of ranks. You don't 'apprentice' kids to Old Fred the village horseshoe maker, not unless you want them to be scullions for the rest of their lives.