Click on the title of the post for a graph from Age Works. What does it say? There are more old people. Much of this increase can be chalked up to the baby-boomers coming of age, and while I'm no demographer, it seems possible that there will be a dip in that percentage, or at least a slow-down in the rate of growth. Of course, this begs the question, what's wrong with an older population?
Naturally, the first point to bring up is the effect on the labor force. 65 year olds aren't quite suited to build bridges, and even non-physical jobs require youth. A 70 year old probably can't put in an effective 20-hour week as an investment banker, while somebody younger can. While these points are somewhat obvious, there is a more subtle aspect to all this. New ideas come from young people, and new ideas lead to innovation. Anyway, I won't belabor the oft explained benefits of a large youth population. Most of the reasons should be obvious.
The next point somewhat stems from the first. Older people, especially if they have retired, "take out" more than they "put in." Ok, when they were young, old people did their share of "putting in," but I'm speaking contemporaneously here. If there is a large old person population relative to youth population, our "taking out" to "putting in" ratio will unfavorable. "Taking out," by the way, means using services like Social Security and Medicare, etc... They cost money, but they are necessary (a debate in itself, but I stand by my point).
Let's say we want to make our population younger, now that we've got a decent grasp on why that would be good. The standard remedy is to usher in more immigrants. The average immigrant's age tends to be lower than that of the overall population. Ok, makes sense, right? Immerse a group with a low average with one with a high average and you should get a new group with an average in between. Well, as before, I'm going to take issue with averages... AGAIN. The U.S. population in 2009, according to the census bureau, was around 307 million. According to the Migration Policy Institute, between 2005-2010 there was a net inflow 950 thousand migrants to the U.S. That is .309% of the U.S. population in 2009. A little hard to make dent in the AVERAGE age with those kinda numbers, yeah?