I’m sure this is an overly simplistic explanation, but I wonder if it spread the way it did among the age groups of those two countries because of how those groups did, or did not, using social distancing. My observation of people in the U.S. is that the fairly young (teens) and the fairly old (60’s and older) are the ones mostly blowing off social distancing. (This is a sweeping generalization with all the caveats that go with that sort of thing.).