I started when I was 10 or 11 by getting into my grandfather's tool box, and using his tools to take apart my bike. Luckily he was around to help me put it back together. He taught me how to oil the chain, to patch a tube (he would never spend money for new tube), and make sure that the bar, seat post, and stem were tight.

Bike mechanics is not rocket science, cooking is a much harder skill to acquire. Bikes and other machines are all "nuts and bolts", when you take something off, put it back where you found it. They are a puzzle of parts which simply go in a particular place in a particular order. If you like puzzles, mechanics is going to be easy.

In my early teens I was already doing long endurance rides, and doing light modifications on my bike. But then I became interested in cars, took auto shop classes in high school, and earned extra money by buying old muscle cars, cleaning them up, and reselling them. I spent a summer as an apprentice working in an auto repair shop, though I found the work dirty and difficult. But I never lost my love of cycling and bicycles. Between high school and university I got a job as a mechanic at a well known bike shop in southern California (despite having no experience at all as a bicycle mechanic). The hours were long, and the pay was awful (you have many more opportunities for career advancement and higher wages at McDonald's), but I got a discount on parts and accessories, and was able to put together a nice bike.

Bicycle mechanics is not difficult, even the newest and most complex bicycles are not far removed from a kid's bike. You can learn all you need to do to perform every repair necessary (short of welding) within a few months. There isn't much I can't do when it comes to bicycle repair, I can build a wheel, I have welded broken dropouts with a car battery and jumper cables.

Spend some time looking at how the bike is put together, noting the hardware which holds it together. Operate the levers, and watch how they move the brakes and derailleurs. Once you understand what everything does, and how it works, you can more easily diagnose problems. When you diagnose a problem, you can probably figure out what is necessary to perform the repair. Doing it right may not be easy the first time, but the best way to learn is by doing.

If you are thinking of becoming a professional bike mechanic, don't bother. Even pro mechanics working for the top European teams are poorly paid. Go to school (or stay in school) and learn something more useful. Minimum wage is fine for part time work when you are a student, but not something you want to be doing for years in the back of a bike shop.