The population of the U.S. will climb to about 370 million in 2080 before reversing course and starting to fall before the turn of the century, according to a new Census Bureau projection.

By 2100, the U.S. will have a population of around 366 million people, unless immigration continues to climb each year, in which case the population could reach as high as 435 million, the bureau forecas

In the implausible event that international immigration were to cease altogether, the population would start to fall next year, dropping to 226 million by the turn of the century.

It’s the first time the Census Bureau has used assumptions on births, deaths and international migration to project as far ahead as the year 2100.

In the bureau’s most likely scenario — it makes several projections based on variable factors — the number of people 65 or older will overtake the number under 18 in the year 2029. By the turn of the century, 29.1% of the population would be in the older age bracket.

“The U.S. has experienced notable shifts in the components of population change over the last five years,” said Bureau demographer Sandra Johnson.

“Some of these, like the increases in mortality caused by the COVID-19 pandemic, are expected to be short-term, while others, including the declines in fertility that have persisted for decades, are likely to continue into the future.”