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…
This is the same sort of thing that a lot of people criticized the FDA over for going after Cheerios for making claims about being healthy for your heart.
http://www.wsj.com/articles/SB124216077825612187
It's a good thing that there are parts of the government out there making sure you can't just make up whatever you want to sell a product.