A lot of this has been posted over and over, but the attendance has to do with the demographic of the city.
Get more people living downtown, within walking distance of the stadium, and you'll see more people there. My season tickets when I lived on East 4th were absolutely amazing.
When people live out in the suburbs, they're less likely to see the hassle of commuting downtown (let alone kids, jobs, times, parking, and the logistics of it all) as worth it. I canceled my season tickets because games just felt more like a hassle than a luxury.
I don't see this as an age thing. Tons of younger people love baseball.
I do see it as a marketing thing. The MLB is the worst run out of the big 3, and it isn't close. They need to promote their personalities, encourage individuals to stand out, and make the game more accessible.