Your question includes an unwarranted assumption: that very few middle-aged or elderly people know about (much less care to join) social media sites like Facebook. That's a common misapprehension among twenty- and thirty-something professionals, who wake up one day to discover that their parents have sent them a “friend” request!
The fact is that, while Facebook doesn't have anything near the market penetration among older people as it does among high-school and college students and young adults, those demographics are changing every day.
Part of what makes Facebook so popular is that it's so easy to use: even a great-grandma can easily create an account and populate her “friends” list with the touch of a button. Whether or not she will be actively engaged on Facebook, constantly updating her status and commenting on friends' posts, will be a matter of her personal proclivities—but keep in mind that retired people can have a lot of time on their hands, and Facebook is an easy way to keep in touch with friends and relatives.
That's why your law firm should have a Facebook presence, even if your clients aren't in the prime of their lives. You'd be surprised at how many people use Facebook to find basic information—such as, say, asking their friends for the name of a reputable lawyer—and you want them to be able to “share” your corporate page easily and quickly. You can even cultivate potential clients by engaging in conversations with them on your (or their) Facebook page, and then inviting them to visit your real-life office.