That being said, there are people who love being told about DF but wouldn't play it. This is to be expected, as DF is a storymaking game where you are encouraged to perceive events within a narrative of your own making for greater enjoyment.
DF appeals to a very specific set of people, and can be broken down even further between the Fortress Mode lovers and the Adventure Mode lovers. I'm the latter. I find Fortress Mode very stressful and annoying, while Adventure Mode qualifies as one of the most modular roguelikes I have ever encountered.
Anyway, your father probably won't ever like DF in its current state because he perceives it as goalless. That's the issue with DF. You have to come to a goal and story of your own within the framework of the existing game. He might like a future-DF where Toady has gotten the game to the point of assisting the player with this task through a more fleshed-out world, but not present-DF.
Age is not only not a factor, older people are going to be a major market for video games eventually. Once you're in your late 70s you won't be doing much skydiving, but you'd have to be nearing death to be too withered for a video game. You don't even have to be able to walk. It isn't much like that right now because the current old people would have to adopt a new form of media to use video games that are mostly focused on a younger demographic, but when the Baby Boomers die and Gen X starts to grey....