You want a nice even spread to make sure that one generation of workers can train, and in some way pay, for the other. The worse your education system the more dependent on the older generation you are.

In Japan I think the problem is a slow rate of birth. So the fear is that the percentage of the population above 65 will increase until it breaks the economy.

Good question. I think the logic is that the most dangerous demographic is twenty something males. If a revolution or riot will happen, it will happen when this group is unhappy. So if they dominate the population in times of severe economic trouble, it's a recipe for big change.

Because those people will need jobs. People in their 20's without jobs have a habit of filling streets and overthrowing governments.

Just cause of the imbalance, I guess. Same problem everywhere in the middle east - you have all these young, smart, energetic people who want to start families and careers, but there are no opportunities available to them. So they get mad and occasionally violent. And I don't blame them, either.