Utah had the lowest median age among the states at 29.2 years. The second-youngest state, Texas, had a median age of 33.6, a whopping 4.4 years older than Utah.
Utah's median age was also 22 percent lower and eight full years younger than the national median of 37.2 years old. Also, Utahns typically were 13.5 years younger than residents of Maine, the oldest state, where the median age was 42.7.
Utah was the nation's youngest even though its median age increased by two years over the past decade from 27.1 to 29.2 years, as its large "echo boom" generation of children of baby boomers aged and started exiting their child-bearing years.
"We're slowly becoming like the rest of the nation," Perlich said. "We are trending down, but we are a generation and a half to two generations behind the national trend on fertility decline."
She added, "If you compare the fertility of Utah women now with where we were in 1980 or 1950, there's been a major decline in fertility. Still, relative to other states, we have a high fertility rate which means larger households. It's all related."
Perlich sees two main reasons why Utah has so many more children than other states. First, is the influence of The Church of Jesus Christ of Latter-day Saints and the value it puts on having children.
Second, "We're younger because we're an in-migration state, even though the migration has slowed. ... Those coming are young people, and young people have babies. And the people who are having babies are having them at higher rates than other states."
She adds that among immigrants are higher numbers of Latinos, and as a group they tend to have a comparatively high fertility rate.
Amid the high fertility rates, Utah's average household size dropped only slightly from 3.13 people in 2000 to 3.10 in 2010.
Again, that number was still far higher than in other states. The No. 2 state, California, had an average household size of 2.9.
The average household size nationally was 2.58 people 17 percent smaller than in Utah. North Dakota had the smallest household size among the states at 2.3, and the District of Columbia was even lower at 2.11.