Three facts about age and the foreign born need to be addressed: the foreign born population is relatively old; the age of new immigrants is increasing, but immigrants form a baby factory.
The foreign born population grew significantly younger between 1960 and today, while the native born population grew significantly older. (See age pyramids here, here and here) Yet the median age of the foreign born is older!
But this is misleading, because it does not take into account the fact that very few immigrants today are very young. Their off-spring are: about one quarter of school age children in the U.S. have at least one foreign born parent.
Thus, to say that foreign born are on average older than native born misses the point that foreign born are responsible for a large number of young people.
The Migration Policy Institute says that many migrants migrate to find work abroad, so a high number in the economically active 20 to 54 age bracket is not uncommon. The majority of immigrants in 2019 were adults between the ages of 20 and 54; hence are of child bearing age. They are birthing boomers. 20% of births are by immigrant mothers, but only 14% of residents are foreign born.
Newly arrived immigrants are getting older.
The average age of newly arrived legal and illegal immigrants was 31 years in 2019, compared to 26 years in 2000. CIS cites several factors likely explain the rising age of new arrivals:
(1) The rise in the age at arrival for immigrants is a broad phenomenon affecting immigrants from most, though not all, of the primary sending regions and top sending countries. Mexico, for example, is undergoing dramatic demographic change, with a rapidly aging population caused by a declining birthrate and improved life expectancy.
(2) An increase in the number of green cards going to the parents of U.S. citizens, and a decline in new illegal immigration relative to earlier years. Family chain migration greatly increased, bringing in parents. Every 100 initiating immigrants from the 1981–1985 admission cohort sponsored an average of 260 family members over the observation period. By comparison, every 100 initiating immigrants from the 1996–2000 cohort sponsored an average 345 family members (here).
(3) Shifts in the unauthorized population. There was an end to the surge of youngish unauthorized persons (mainly men), which began in the late 1980s and stopped in 2007-2008. In 2018, the percentage of unauthorized men over 44 was 23%, while 34% of the total U.S. population was over 44. The unauthorized population in the U.S. declined by about 15% since 2008 while the total population grew by 9%. The unauthorized population fell from 30% of total foreign born in 2008 to 23% in 2020.