"I am 98 years old, son. Do you expect me to have an email account? I do have a Facebook account, though."
Lumping all elderly in the non-computer-savvy category might be too harsh. However, there are some people who just don't prefer to keep up with the times. So, reserve your paper invites for them. As far as sending an invite on Facebook is concerned, that would be taking this thing a bit too far, even according to the online invite proponents.