"Demographers have known for a long time that, absent truly gargantuan and ever-increasing rates of immigration, it isn't actually possible for immigrants to undo or dramatically slow the overall aging of society.
...The reason behind this truth is simple: Immigrants are human beings, not just the idealized workers or child-bearers that some commentators imagine. As humans, they immigrate at all ages, grow old over time, and are choosing to have smaller families. As a result, they add to the population across the age distribution and do not fundamentally change the nation's age structure.
...One can advocate for immigration for any number of reasons, including the fact that immigrants themselves benefit greatly by coming here. But it is simply dishonest, and therefore irresponsible, to claim that immigration will address the fiscal and other challenges of an aging society that maintains an enormous welfare state for the elderly."