Yes the problem is that dwarves are able to do certain difficult and dangerous tasks even when completely untrained. However I like to think of the education as being abstracted out of the game, the reason they are able to do those tasks is because they were given basic education to so them as children.
This is Dwarf Fortress. What is this "abstraction" you speak of?
Not doing things in the most detailed and complex, computationally brute-force way possible? Are you an elf or something?
I do not understand what is ridiculous about 12 year-old marriage or elves taking the normal amount of time to grow up. Marrying is not something that requires prior education, merely sexual impulses which 12 year-old's generally have. How fast elf children grow up is completely different from how long elves live because how quickly a creature reaches maturity has nothing to do with it's longevity.
Generally speaking, even those 12-year-olds who actually
have started menstruation (and while some do, plenty don't come around to it until 15 or so - there's a reason 16 is considered a woman's "coming of age" year,) will not be able to carry children as healthy or as safely to term as an adult.
Having them spit out babies like a machine gun at 12 years of age onwards IS pretty ridiculous. With medieval-level medicine at their disposal, you could expect at least 80% of them to die in childbirth from complications before they really became adults.
We do not need stages of adolescence nor of childhood (not saying this is a bad idea) in order to implement the idea of a minimum age for jobs to be done at. We just set the jobs the creature-type is actually capable physically of performing and the minimum age at which they can do them.
And how do you expect players to recognize when they have attained those ages?
Growth is divorced from life stages. Life stages exist solely as a means of dictating what jobs/actions are available, and being a label for players to easily see what age they are. If you break up what ages jobs become available, there's no reason
not to use a label that tells the player what jobs the child is capable of taking.
The idea of apprentices is to make children useful and also allow them to reach adulthood with skills that allow them to match those immigrant dwarves that arrive. Apprentices do not reduce productivity, instead they increase the productivity of the dwarf that they are attached to by doing various abstracted away simple menial tasks (not hauling) that dwarf would normally do themselves as part of the job but do not require special skills.
Translation: They do nothing.
There are no other tasks to "abstract" in a workshop, dwarves just use the boulder or block they used to create a workshop to mold stone/wood/whatever with the power of their minds/beards. Time spent at a workshop is always trivial compared to hauling, and as such, any other task is nonexistent.
Their skill gain is capped at half of the skill of their master (no 12 year old legendary dwarves) and once they reach adulthood they become workers in the normal sense, able to do all the jobs that the other adult dwarves with the same skills can do.
Translation: All kids in your fortress will reach legendary (because your skill ranks can go up thousands of levels above legendary, and legendary workers are set to work round-the-clock creating vendor trash) in at least one skill, while all dwarves outside your fortress are stuck as novices and dabblers.