The age structure of achievement is being ratcheted upward, due to specialization and the growth of knowledge. Mathematicians used to prove theorems at age 20, now it happens at age 30, because there is so much to learn along the way. If you are a smart 22-year-old, just out of Harvard, you probably cannot walk into a widget factory and quickly design a better machine. (Note that in “immature” economic sectors, such as social networks circa 2006, young people can and do make immediate significant contributions and indeed they dominated the sector.) Yet you and your parents expect you to earn a high income — now — and to affiliate with other smart, highly educated people, maybe even marry one of them. It won’t work to move to Dayton and spend four years studying widget machines.
You will seek out jobs which reward a high “G factor,” or high general intelligence. That means finance, law, and consulting. You are productive fairly quickly, you make good contacts with other smart people, and you can demonstrate that you are smart, for future employment prospects.
The rest of the world is increasingly specialized, so the returns to your general intelligence, as a complementary factor, are growing too, in spite of your lack of widget knowledge. “Hey you, think about what you are doing! Are you sure? How about this?” often sounds bogus to outsiders but every now and then it pays off and generates a high expected marginal product.
Both supply and demand sustain this Smithian equilibrium.
There are other factors of relevance, as explained over a very good session last night; the people there comprised about half of my Twitter feed.