Of all the numerous kinds of employment discrimination, hiring younger workers in preference to older ones earns the least public condemnation. That's a growing problem, not just because equality is increasingly prominent on the political agenda but because ageism makes less and less economic sense as birth rates go down and nations age.

Since 1950, the median age of the population has increased by 47 percent in Europe and by 29 percent in North America, and the trend isn't about to change, according to the United Nations Population Division. In the coming decades, the United Nations expects Asia and Latin America to age faster than today's rich nations of the West.

Though it imposes a bigger burden on social security systems, the aging isn't all bad — humankind is getting better at fighting diseases and making life easier, so people live, and are productive, longer than before. A longer working life, stretching well past the age of 60, is increasingly the norm. But the share of people over the age of 55 who are gainfully employed has been rising faster in some countries with aging populations and slower in others.