William Knudsen was President of General Motors prior to the start of World War II, so I'm assuming this was written pre-war or even during the war. The last paragraph could have been addressing the the need to re-tool after the war but it seems also to apply to our current situation in manufacturing, especially if you use mechanic in the general sense of the word rather than the specific sense as someone who works on wheeled vehicles. I especially like the line "best of all is the man who combines the learning of books with the learning which comes of doing things with the hands."
When I was 21 I was going to college, on and off, I had a good start as a welder, was able to swap engines and keep my cars and bikes running. Nothing unusual back then. Pretty much the same as everyone else I knew, unless you got drafted. Now days, if a young guy will get up off the couch and decide to be a mechanic, again in the generic sense, the sky's the limit. CNC machining, CAD drafting, 3D printing, Programmable Logic Controllers - all kinds of computer driven opportunities, and with maker spaces, YouTube videos, and web sites like Coursera, you can take college courses free from some of the finest universities in the world.
Twenty one or not, it's a great time to be a mechanic.