Bar chart showing the change in the population under 18 years old by county in the Austin, Texas, area. From 2010 to 2020, the population of young people in Hays County grew by 43%, the most in the area, followed by Williamson County with a growth of 28%. The under-18 population grew by 24% in Bastrop County, 18% in Caldwell, and 12% in Travis.