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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E-mail: lee@desnews.com