Recently released U.S. census data shows that Montana’s largest counties shifted demographically older from 2020 to 2025 as their population of residents age 65 and older grew relative to their population of children.This Montana Free Press analysis uses an aging index to compare the state’s po pulation of older residents with its population of children. The index shows the number of Montana residents age 65 and older for every 100 residents under 18. A higher aging index reflects a population in which older adults make up a larger share of the overall population relative to children.Statewide, Montana’s aging index rose from 93 older residents per 100 children in 2020 to 110 per 100 children in 2025. The state added more than 36,000 residents age 65 and older during that period, while the number of children declined by about 2,500. Those changes align with statewide trends the Legislative Fiscal Division flagged in a report to lawmakers in late June. The index increased in all eight of Montana’s most populous counties over the five-year period. But the shift didn’t occur in the same way in each location. In some counties, the change was driven by growth in the older population combined with a decline in the number of children. In others, both groups grew, but the older population grew faster. Missoula County had the largest shift over the period. In 2020, its aging index was about 91 older residents per 100 children. By 2025, older residents outnumbered children, with the index rising to about 112. That change was driven by Missoula adding nearly 3,500 residents over the age of 65, while the number of children under 18 declined by more than 1,100.Lewis and Clark County followed closely behind, moving from about 91 older residents per 100 children in 2020 to 110 in 2025. The county added about 2,700 older residents during the period, while its under-18 population declined slightly.Flathead County also aged, but less steeply than Missoula and Lewis and Clark counties. Flathead County experienced rapid net growth during the period, with Kalispell experiencing the fastest population growth in the state for multiple years. Of the more than 10,000 new residents added to the county, more than 4,000 were 65 or older. But the county also added 1,500 children, helping keep Flathead’s age shift smaller than in several other large Montana counties.As of 2025, Ravalli County remained the oldest of the eight. It started with an aging index of 146 older residents per 100 children in 2020 and rose to 163 by 2025. Unlike several other large Montana counties, Ravalli also added children, but its older population outgrew the younger population enough to push the index higher.Butte-Silver Bow County joined Ravalli as the only counties among the eight where older residents already outnumbered children in 2020. Butte-Silver Bow’s index rose from 103 to 120 older residents per 100 children, leaving it as the state’s second-oldest large county by age index in 2025.Gallatin County remained the youngest of the state’s eight largest counties, despite registering the largest percentage increase in its age index. It rose from 68 older residents for every 100 children in 2020 to 84 in 2025.  Yellowstone County rose from 78 to 93, while Cascade County had the smallest increase, moving from 86 to 99.  The post Montana’s most populous counties have grown older, census data shows appeared first on Montana Free Press. ...read more read less