The Center for Immigration Studies has released a report on the way in which immigration will affect the national population. Especially interesting is, I think, its section on the way in which immigration can affect the working-age population.
This report suggests that increasing immigration levels has relatively little effect on the percentage of the working-age population and challenges the assumption that immigration alone can compensate for some of the pressures faced by an aging society.
For example, based on Table 5 of this report, increasing the net immigration rate to 10 million people per year (the current net level is 1.25 million) would lead to 62% of the US population being between 15 and 64 in 2060; having 0 net immigration would lead to 57.2% of the US population being between 15 and 64 in 2060. Currently, 67.3% of the US pop. is between 15 and 64. So even with 10 million new people coming in per year, the percentage of people aged 15-64 would still substantially decrease. A net immigration rate of 3 million/year would lead to 60% of the US population being between 15 and 64 in 2060 and an overall national population of over 615 million by 2060 (see Table 3).
Presuming a retirement age of 64, the ratio of workers to retired-aged persons is currently 5.4. Under 10 million immigrants/year, that ratio would decline to 3.8 by 2060 (it would be 2.4 under 0 immigrants/year). So while immigration rates do have some effect on the worker-retired ratio, even the (by current standards) high rate of 10 million/year would not prevent a somewhat substantial decline in this ratio.
The report also has an interesting table on the effects of raising the retirement age on the worker-retired ratio. Presuming a net immigration rate of zero, this table says that raising the retirement age to 68 would lead, by 2060, to a worker-retired ratio of 3.0, the same number as a net immigration rate of 2.5 million/year (double the current rate) would lead to if the retirement age were kept at 65. In order to get to a 3.1 ratio, the net immigration rate would have to add another 500,000 (to 3 mil./year). By 2060, we will have a 2.7 worker-retired ratio (w/retirement age of 65) under the current net immigration level of 1.25 million/year.
This report claims that high levels of immigration have a limited impact on the aging of society. It even observes that, in 2006, the average immigrant's age was 40; the average native-born age was 36.