The way I see it, they tapped into a widely held belief , rather than fears. It is a popular belief that doing mental work staves off cognitive decline (including age-related). I bet you that everyone at Lumosity actually believes it because their mother told them that when they were kids. My wife (Japanese) always says "good for older people" about any sort of puzzle. She didn't get this from being deceived by Lumos…
Exactly. Want to tell carrots? Go ahead. Want to sell carrots by telling people with failing sight that it will improve their vision, and talk them into a monthly fee for carrot delivery? You'd better have some research to back that up.