Maybe Utah should be called "Youth-tah."

Once again, new census estimates peg Utah's population as the youngest among the states. And Utah is also home to eight of the 50 youngest-population counties in America (out of more than 3,100 counties nationwide).

Estimates say that nearly a third of Utah's residents are under age 18, and one of every 10 residents is under age 5 — both tops in America. Those numbers are both about 40 percent higher than the national average.

"We have a large number of women who are in their child-bearing years. And they have more children than average. Those two things combined mean we have a young population," said Utah state demographer Juliette Tennert.

"Some of the things that fuel that are that the LDS Church emphasizes the importance of family, and people here marry younger and have bigger families," she said. Maybe that also shows that Utah's founder, Brigham Young, had an appropriate last name for the demographic legacy created largely by members of his church.

Tennert said the average Utah woman has 2.5 children in her lifetime, compared to an average of 2.1 nationally. The average Utah woman first marries at age 22.7, compared to the national average of 25.9.

University of Utah research economist Pam Perlich adds that high fertility rates have been sustained in Utah for generations. "So that means every generation is larger than the last" — and it produces a large, young population.

Also, Perlich notes another reason for Utah's youthfulness. "Over the long term, Utah has been a net in-migration state. It has had economic growth to support more population," and thus attracted immigrants seeking work. She said most of those immigrants have been in their child-bearing years.

She says such immigration has included large numbers of Hispanics recently, and that is also a key "because Hispanic women in the state have a fertility rate that is a full point higher than non-Hispanic, white women." It also means that younger people in Utah are much more ethnically and religiously diverse than its older generations. "I call it the new Utah," Perlich said.

Census estimates released last week for 2007 show about 30.9 percent of Utah residents are younger than 18 years old. Nationally, only 24.5 percent of residents are under age 18.

Also according to compiled county-by-county estimates nationally, about 9.7 percent of Utah's resident population is under age (also tops in the nation), compared to a national average of 6.9 percent.

Many Utah counties are among the nation's youngest.

When using median age as the measure, Utah County is No. 9 among the 3,100-plus counties in America with a median age of 24.4 years.