Immigrants are not so much younger than the non-immigrant population than they are child-bearers more than non-immigrants. That way, they mitigate the aging of the population.

The nation’s median age increased by 0.2 years to 38.9 years between 2021 and 2022, according to the U.S. Census Bureau.

The median age in the United States in 1990 was 32.9; in 2000, 35.3, and in 2010, 37.2

The median age has been rising because the birth rate has been dropping—1990: 16.7 births per 1,000 population. 2000: 14.0 births per 1,000 population. 2010. 13.0 births per 1,000 population. 11.9 births per 1,000 population.

The birth rate has declined in large measure due to our not having a fertility rate at replacement (2.1) or higher since the 1970s.

How immigration fits in: Immigration probably does not lower the median age, or if so negligibly. What it does do is produce a lot of babies., Foreign born persons make up 14% of the population but produce 20% of all babies. Why? Because foreign born persons are relatively more concentrated in child-bearing cohorts (young adults and middle age). See my analysis of immigrants as a birthing factory here.