Population Characteristics - Age
(2018)
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| County Name | Under 18
Rank | Under 18 | 18 - 64 | 65 and over | Dependency ratio |
| Jefferson | 1 | 34.1% | 54.4% | 11.5% | 83.8 |
| Franklin | 2 | 31.9% | 53.9% | 14.2% | 85.4 |
| Cassia | 3 | 31.7% | 54.1% | 14.2% | 85.0 |
| Power | 4 | 31.2% | 53.3% | 15.4% | 87.5 |
| Jerome | 5 | 30.8% | 56.2% | 13.0% | 77.8 |
| Bingham | 6 | 30.8% | 55.0% | 14.3% | 82.0 |
| Bonneville | 7 | 30.8% | 55.9% | 13.3% | 78.8 |
| Lincoln | 8 | 28.9% | 57.0% | 14.1% | 75.6 |
| Caribou | 9 | 28.6% | 53.9% | 17.5% | 85.5 |
| Canyon | 10 | 28.5% | 57.8% | 13.8% | 73.1 |
| Minidoka | 11 | 28.4% | 55.4% | 16.3% | 80.6 |
| Oneida | 12 | 28.3% | 51.4% | 20.3% | 94.5 |
| Twin Falls | 13 | 27.5% | 57.1% | 15.4% | 75.1 |
| Gooding | 14 | 27.5% | 55.2% | 17.3% | 81.0 |
| Madison | 15 | 26.9% | 66.2% | 7.0% | 51.2 |
| Bear Lake | 16 | 26.7% | 52.6% | 20.6% | 90.0 |
| Bannock | 17 | 26.2% | 59.5% | 14.2% | 67.9 |
| Payette | 18 | 26.2% | 55.4% | 18.4% | 80.5 |
| Owyhee | 19 | 26.0% | 56.2% | 17.8% | 77.9 |
| Fremont | 20 | 25.7% | 57.4% | 16.8% | 74.1 |
| Elmore | 21 | 25.5% | 61.0% | 13.5% | 63.9 |
| Teton | 22 | 24.6% | 62.9% | 12.5% | 59.0 |
| Butte | 23 | 24.0% | 52.6% | 23.4% | 90.2 |
| Ada | 24 | 23.8% | 61.8% | 14.4% | 61.8 |
| Camas | 25 | 23.3% | 54.2% | 22.4% | 84.5 |
| Clark | 26 | 23.2% | 59.0% | 17.7% | 69.4 |
| Boundary | 27 | 23.1% | 54.4% | 22.5% | 83.9 |
| Gem | 28 | 22.8% | 55.2% | 21.9% | 81.0 |
| Kootenai | 29 | 22.8% | 58.2% | 19.0% | 71.8 |
| Washington | 30 | 22.7% | 52.2% | 25.1% | 91.6 |
| Lewis | 31 | 22.6% | 51.0% | 26.4% | 96.1 |
| Benewah | 32 | 22.6% | 54.9% | 22.5% | 82.0 |
| Blaine | 33 | 22.1% | 58.3% | 19.6% | 71.5 |
| Nez Perce | 34 | 21.2% | 58.8% | 19.9% | 69.9 |
| Shoshone | 35 | 20.6% | 56.6% | 22.8% | 76.7 |
| Idaho | 36 | 19.7% | 52.4% | 27.9% | 90.9 |
| Bonner | 37 | 19.7% | 55.8% | 24.5% | 79.3 |
| Latah | 38 | 19.0% | 67.5% | 13.5% | 48.1 |
| Lemhi | 39 | 18.7% | 51.0% | 30.3% | 95.9 |
| Valley | 40 | 18.1% | 56.8% | 25.1% | 76.1 |
| Adams | 41 | 17.7% | 53.5% | 28.7% | 86.8 |
| Custer | 42 | 17.4% | 53.6% | 29.0% | 86.6 |
| Boise | 43 | 16.9% | 58.0% | 25.1% | 72.6 |
| Clearwater | 44 | 16.6% | 56.1% | 27.3% | 78.3 |
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