Why youth bulges are dangerous

According to a report by Population Action International, young adults are correlated with civil conflict:

Countries in which young adults made up a large proportion of the adult population — 40 percent or more — were more than twice as likely to experience an outbreak of civil conflict during the 1990s as those below this benchmark. These youth-bulge countries are in the developing world, where youth unemployment rates are generally three to five times that of adults.
High fertility rates coupled with declining infant mortality are the major reason for high proportions of young adults. In East Asia, proportions of young adults began to decline significantly less than two decades after fertility began its own fall.

Immature frontal lobe development seems to be a likely neurological candidate explanation. According to thie piece on teenage brains, full maturation of self-control doesn't happen until people reach their mid-20s:

Ron Dahl, a pediatrician and child psychiatric researcher at the University of Pittsburgh Medical Center, says a desire for thrills and taking risks is a building block of adolescence. The frontal lobes help put the brakes on such behavior, but they're also one of the last areas of the brain to develop fully. Located right behind the forehead, the frontal lobes actually grow larger than adult size in puberty. But the process is far from complete; refinement of the frontal lobes can continue into the early 20s.

Thus, the young would tend to have a lower threshold for violence, as their brains literally have fewer checks and balances.