From the most recent Congressional Budget Office forecast to 2053: Average net immigration per year will 0.31%. Average growth of 25-54 population will be 0.2% In the past 40 years, that prime working age cohort grew by 0.9% per year. Hence, Immigration growth is expected to be 0.31/0.2 = 50% faster than the total prime working age population.
Persons 65 and older to increase by 1.2% per year, much faster than the prime working populatio. The fertility rate will be at 1.75 per woman.
Total population will increase by 0.4%, lifted by immigration and longer life expectancy.
The contribution of immigration above is understated because it does not take into account that new immigrants are more concentrated in child-bearing ages. One needs to look at the burgeoning second generation population. Recent immigrants comprise a birthing factory. Immigrants account for 20% of all births, even though at present the entire immigration population is just 14% of total population. By the mid Century the second generation will start to have their own children.
Go here for the CBO forecast.