The nation’s adult population increased faster than the under-age-18 population and the U.S. population as a whole in the last decade, according to the first detailed 2020 Census data released today.

In 2020, the U.S. Census Bureau counted 331.4 million people living in the United States; more than three-quarters (77.9%) or 258.3 million were adults, 18 years or older — a 10.1% increase from 234.6 million in 2010. The aging of baby boomers, those born between 1946 and 1964, who were ages 57 to 75 in 2021, is partly driving the growth in the adult population.

By comparison, the younger population under age 18 numbered 73.1 million, or 22.1% of the U.S. population in 2020, a 1.4% decrease from 74.2 million in 2010. The slow decline of the younger population is in part due to a general decrease in fertility, ongoing since 2007.

During the same period, the total U.S. population grew at a slower rate than the adult population: up 7.4% from 308.7 million in 2010 (Table 1).

Although the adult population grew from 2010 to 2020, the growth was lower than in the first decade of the 21st century when it grew 12.2%, to 234.6 million in 2010 from 209.1 million in 2000.

The 2020 Census shows that the Northeast had the largest proportion of adult population (79.7%) and the smallest proportion of people under age 18 (20.3%) (Table 2a).

In contrast, the South had the lowest proportion of adult population (77.5%) and the highest proportion of young population (22.5%).

But only the South maintained growth in both age groups from 2000 to 2010 and 2010 to 2020: The young population in that region increased by 2.2 million (8.7%) and by 0.6 million (2.1%), respectively (Table 2b).

The adult population in the South also experienced the largest numeric increase of 11.1 million, or 12.8%, between 2010 and 2020.

The figure below shows that the adult population grew in all regions during the 2000s and 2010s. The West experienced the fastest population change between 2010 and 2020, adding 7 million (13%) adults.

However, the Northeast was the only region where the adult population grew faster between 2010 and 2020 (6.8% or 2.9 million) than between 2000 and 2010 (6% or 2.4 million).

In the rest of the regions and across the country, the adult population grew at a slower pace over the last decade. The younger population also grew more slowly or declined.

Use the interactive data visualization below to learn about the size, growth rate and distribution of the adult and under-age-18 populations in your state and county.

Stella Ogunwole, Megan Rabe and Zoe Caplan are demographic statisticians in the Sex and Age Statistics Branch in the Census Bureau’s Population Division.

Andrew Roberts is chief of the Sex and Age Statistics Branch.