Most older people seem to think piracy=stealing, and I suspect any argument designed to make a distinction will probably just steel their resolve.
The only way to explain how its bad is to latch onto examples of things that were made possible by the DMCA, and would no longer be possible under SOPA. So I would focus on internet stuff most people DO know about. Basically what you said about reddit, but applied to even more mainstream 2.0 sites. The obvious ones-
"If SOPA had been in place, youtube would never have existed, and if SOPA passes, youtube will probably shut down."
"If SOPA had been in place, WIKIPEDIA could easily have been shut down for something as simple as a user uploading a picture of a celebrity for their entry"
Those sites are big enough that they're known even by older people. At the very least they've heard about them. And if not you can talk about how amazing they are to warm them up.
Then, you can explain that wikipedia has millions and millions of entries by tens of thousands of people, and if the laws are that brittle and unforgiving, collaboration on that scale simply becomes impossible.