I don't think it is the sole reason but they ARE representative of the voting US population. The 65+ demographic has the highest voter turnout within their age group.

In my country, voting is almost entirely the domain of the elderly. I can count on one hand the number of people in my age range I know that actually go out and vote.

I think there is also some portion of belief that "this person has a lot of experience so must be better suited for the job".