The U.S. population is aging and the ranks of men are growing slightly faster than those of women, the Census Bureau said.

While the nationwide median age is 37.2 years, populations in seven of the 50 states now have a median age of 40-plus, the Bureau said, based on an analysis of the 2010 Census data.

The national median age in 2000 was 35.3 years.

The number of men in the United States increased by 9.9 percent between 2000 and 2010, compared with a 9.5 advance for women.

Yet women still remain in the majority, numbering 157 million and representing 50.8 percent of the total population. Last year, there were 96.7 men for every 100 women, up slightly from 96.3 per 100 in 2000.

Demographers suggest the narrowing of the male-female population gap is due to a growing incidence among women of lung cancer and stress-related health problems.

The 45-64 population increased 31.5 percent to 81.5 million over the last decade, owing mainly to the aging of the baby boom generation, the Census Bureau said. While the number of people 65 and older increased by 15.1 percent, to 40.3 million.

Growth in the under-18 population was only 2.6 percent between 2000 and 2010, though minors still constitute 24 percent of the U.S. population.

People between the ages of 18 and 44 make up the largest cohort, 36.5 percent, but their ranks expanded by a mere 0.6 percent in the 2000-2010 period.