I was 18 when I started working on vehicles. The turning point for me was when I took my car in for a tune up, got charged for it, and found out nothing was replaced, nothing. They did refund my money, but felt it was time to get my hands dirty and make sure things were done the way they should be, even rebuilding my own engines.
We had an Excellent Mechanics teacher at the school I attended and the only reason why I could do the work myself, he was a wealth of knowledge when it came to engines and performance and the spring board for me to be able to work on Cars, Motorcycles, Fixed Wing Aircraft piston (as assistant) and Helicopters piston and some light turbine work which is a different beast altogether (as assistant).
Actually I had some great mentors through my walk in life and one of the reasons why I like to help here when I can, is because of those people that helped me. You never stop learning when it comes to trouble shooting and repairs. I guess thats enough of the "Ramble Channel", switching off.