I think that late 1950's peak shows the post WW2 boom whose young'ins grew up around bikes, especially those of the slightly older guys born right after the war. My example is my older brother's group, born in '46. That group might be the largest, but some are probably done riding now. In 10 years, the '60s-born will be the largest. Think about it; what was the last demographic that had access to mini-bikes and such like we had?