Isn't it hard to conclude that this is causal? How do we know we're not just seeing that the set of people who elect to take on the responsibility of walking, feeding, and socializing with a dog are likely to see a slower cognitive decline? That they take on dog ownership because they're sharp and capable -- not the other way round.

Intuitively, I do believe that being forced to walk a dog every day when you might not have otherwise exercised, will come with health benefits. And it does not seem implausible for their findings to be true. But did they actually prove anything here?