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.

Cache County ranks No. 12 at 24.9 years of median age; Iron County is No. 16 at 25.6; Sanpete County is No. 33 at 27.5; Juab County is No. 35 at 27.6; Tooele and Davis counties are tied at No. 42 at 27.9 years; and Morgan County is No. 50 at 28.2 years.

When the percent of population that is under age 5 is used, Utah County is No. 7 at 11.5 percent; Cache County is No. 19 at 10.6 percent; Iron County is No. 26 at 10.14 percent; Tooele County is No. 22 at 10.12 percent; Davis County is No. 30 at 10.01 percent; Duchesne County is No. 36 at 9.79 percent; Wasatch County is No. 44 at 9.74 percent; and Juab is No. 50 at 9.58 percent.

In both measures, Utah has eight of the top 50 counties — but a slightly different group of counties is on each list. Utah, Cache, Iron, Tooele, Davis and Juab counties made both lists. Sanpete, Wasatch, Duchesne, and Morgan made one each.

While it did not appear on either list, Salt Lake County is No. 84 nationally by percent of population age 5 and under at 9.1 percent, and No. 130 nationally by youngest median age at 30.8 years. Also, Weber County is No. 106 nationally by percent of populations age 5 and under at 8.91 percent, and No. 111 by youngest median age at 30.3 years.

Perlich said Utah, Cache and Iron counties may be the youngest in the state in part because of the students at large universities in each — Brigham Young University in Provo, Utah State University in Logan and Southern Utah University in Cedar City.

"Where you have the presence of a university and the dominance of the LDS culture, young men leave for a mission at age 19 or 20, come back and have families while still in the university," she said.

"They don't necessarily stay after graduation. But there is always a large number of infants per capita in that (Utah) County (for example) ... There is a permanent overrepresentation of infants in the population," she said.

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