A friendly reminder that correlation does not imply causation. There is no contradiction between this result being real, and meditation being absolutely worthless at preventing loss of grey matter.

This is a small sample, and it's just as plausible that (I'm making this up) vegetarians are more likely to meditate, and a vegetarian diet correlates with more grey matter.

If this is a significant result, the takeaway isn't that "I should meditate to keep my grey matter", but rather "I should be the kind of person that would have meditated in this study if I want to keep my grey matter".