...is just as useless as in younger people.
I would tell them it is a huge scam with no plausible mechanism of action, and not a shred of credible evidence that it does anything. Since seniors are fairly trusting and gullible, they may get fooled by a deceptive marketing campaign, so I'm sure as an honest and moral person, you wouldn't want to take advantage of someone's granny, would you?
It is nothing more than an expensive foot massage.