There are too many smart people for too few smarts-required jobs, just like there've never been enough English professorships for all the English majors who want to teach.
My instinct is to say that this cannot happen, because wages would adjust. With English professorships, I think I can tell a story in which the cartel/guild of English professors is able to earn economic rents (excess wages), which creates an excess supply. Control over the tenure process is what does the trick.
But the problem of twenty-somethings doing jobs that are beneath their level of intelligence seems like a market failure of sorts. So, In Tyler Cowen-like fashion, let me toss out a few hypotheses.
1. Perhaps the labor market matching process works poorly when you first go on the job market. It could be that the errors are symmetric: there may be as many twenty-somethings who are too dumb for their jobs as vice-versa.
2. College courses provide few useful job skills, and employers know this. That is why they bring college students in at low wages. Those who actually turn out to have job skills (e.g., because they are adept with computers) are exceptions, and they either get quickly promoted internally or move on to better things elsewhere.
3. New employees over-rate their computer skills and under-rate information technology bureaucracies. I did find in my career that developing a computer system for multiple types of users that can be modified by several people is a very different problem than writing a program to solve a problem for yourself.
4. The wage distribution is shifting, because of what Dan Pink calls the three A's: abundance, Asia, and automation. We need more super-smart people and more not-so-smart people, but not so many medium-smart people. I've never bought this story, but there's a book about it (or two, if you count Dan's). And even if it's true, it should lead to lower wages for medium-smart people, but not to under-utilization of their abilities.
5. There are more dumb bosses nowadays, because technology has passed them by. In a ruthlessly competitive market, they should get winnowed out. But not everybody is in a ruthlessly competitive market--my daughter's job was with a nonprofit.
6. It takes a lot of effort to challenge employees. It's less risky and less time-consuming to give them scut work. Although giving an employee an opportunity for growth and challenge increases the probability that the employee will stay, it does not eliminate turnover altogether. Young people are notoriously unstable, so that it may not pay to invest in someone unless they prove they are going to stick around.
By the way, I forgot to thank Future Tense for the pointer to the post that started this topic.