I think the Kinsey 10% rate came from asking if respondents ever had a same-sex experience. And it was done just a few years after WWII, when many respondents were veterans who had lived in same-sex living quarters during the war.
I bet older people won't admit it because of social pressures when they were growing up. Now that it's much more acceptable, younger people don't have as much stigma associated with coming out and are willing to admit it.