In the US, and Canada, there is actually not much difference in
height between someone of 45 and someone of 18,
besides the 0.5-1 inch lost to aging.
In Europe, China, Korea, there can be a big difference
since nutrition, both in terms of calories consumed and
nutriments and general health (how diseased you are
growing up), is representative of how much of your
genetic height potential you will reach.
But, once you've reached that potential, that's it.
That's why the younger generation, at least here,
is NOT taller than the previous one. Genetics
don't change in a generation, only its gene expression
(through environmental factors) can change.