A watchmaker or clockmaker would typically serve an apprenticeship of around seven years. That depended on the market: in Geneva, clockmaking in the later 18th c. was so lucrative that parents would pay for the sons to be apprenticed to a good maker, and the apprenticeship period might be longer. That's because in the pre-industrial world it was possible to hold someone from the age of 14-21. They would work and learn for a few years, and then, trained, be skilled labor working for room and board for several more. If they learned and then fled before their indenture was up, they could be pursued, captured and returned. But by the 1890's the apprenticeship system was pretty much gone. Industrialization and cheaper power sources made that trade-off of labor for training increasingly pointless, and laws had been changed so that it was impossible for you or your parents to put you into bondage. Workers were just trained, either on the job or in schools ( like Parsons Horological School) , and then employed. The hand powered wheel-cutting engines of the 18th c. were replaced in the 1870's by steam powered presses with dies, and the gears finished by other machines. So, in that respect, someone 20-22 years old could indeed be trained to work on clocks ( or a bright one with a mechanical aptitude like Henry Ford might teach himself or take a correspondence course). But the norm was to employ someone earlier:

Look at John Woodruff; he was a young man then of nineteen. When he first came towork for me at the age of fifteen, I believed that he was destined to bea leading man. He is now in Congress (elected for the second time,)honest, kind, gentlemanly, and respected in Congress and out of Congress. Look at him, young men, and pattern after him, you can see inhis case what honesty, industry and perseverance will accomplish.

Chauncey Jerome : History of the American Clock Business for the Past Sixty Years

David S Landes: Revolution in Time