Perhaps the Torah wants us to realize that only the young, those still pure and uncontaminated, are worthy of such a task. They still have ideals that have not been allowed to deteriorate in the face of life's practicalities and difficulties. Thus their approach to building a Mishkan will of necessity be less tainted and conflicted than that of the older, wiser but more battered adults.
It's an interesting idea: teens are well known for often being somewhat prophetic in their zeal - I can think of many times that a Youth Grouper got obsessed with some issue of justice - from Refusenicks in the Soviet Union to the synagogue using non-eco-friendly coffee - and just wouldn't let go of it. They went after their cause with an almost maniacal devotion. It's easy, as adults, to see that zeal as naive, or immature, or somehow inappropriate. But, it's worth remembering that there is something very pure, and very powerful, in that kind of zeal. Idealism without realism might not be the most effective thing in the world, but it certainly has its place.
How interesting would it be if the Midrash was right, and God was, in fact, saying that the only one worthy of building the mishkan was someone who had that fire, and that zeal. Like I said in my last post, I keep talking about the danger of single-minded zeal (and, I still believe all of that). But, as always, there's something to the other side. Or, to put it a bit differently, and risk driving myself crazy, maybe we shouldn't be too fanatical about opposing fanaticism?
[By the way, if you find this interesting, try to find a copy of the essay Priest and Prophet by Ahad HaAm. It's a classic piece which explores the relationship between Prophets - people who are zealously devoted to core principles, and refuse to accept any compromise in them - and Priests - people who represent the institutions of society, and are always willing to seek compromise to allow principles to be acted out in reality. Ahad HaAm's thesis is that a healthy society needs both - without Priests, nothing ever gets done; without prophets, compromise goes too far, we stray from our core principles and nothing of worth ever gets done!]