It's becoming quite strange--my parents are in their late-50's and they're becoming huge fans, along with many other people whom I would never have guessed would like the show.
I'm 55 and my wife is 56. We love it because we've both been Trekkies and sci fi fans our whole lives, and we love the genre references and jokes. We think of it was a sitcom written "for us nerds."
The strange part isn't their age--it's that my parents are not sci-fi fans in the least, and have little-to-no knowledge of higher mathematics or physics.
My siblings gave me hell about my affinity for sci-fi when we were kids, and now they watch, too.