Could it have something to do with wealth and seniors generally having more of it? And I think in the US elderly vote far more than young people do, and they're probably not voting for young people.
It mostly has to do with the party structure. The older you are the more time you have to build up power within the party, which makes it easier to win the nomination. The only people who really break through this do so by having a disruptive, anti-establishment message, like Obama. JFK is another example of this. Trump's shtick also could have been pulled off by a younger person; he partly leveraged his preexisting brand to outflank establishment Republicans on issues that the base cared about (outsourcing, immigration, China, Middle East wars). Because the Democrats have a younger, more radical element, their primary structure also has built-in mechanisms to suppress candidates who appeal to that element. Republicans were kind of taken by surprise by Trump, the relatively weak measures that they used to marginalize a figure like Ron Paul wouldn't work on Trump because he was already famous, and utilized his rallies and the internet to bypass official channels of communication like debates and talk directly to the base. He also knows how to monopolize media attention.
This is also part of why US national politics are so corrupt. People come to D. C. with big dreams and grand ideas, but by the time it's 'their turn' to take a swing at the plate they've been ground down by years of party politics to be compliant to party demands (which are essentially corporate demands).