The question is whether it is cause or effect. Vitamin D levels are directly related to sun exposure and healthy elderly get more sun than the infirmed.
It would be interesting to see if a double blind study with Vitamin D supplementation has the same effect.
I am mid 50’s, in the sun maybe hours per day, and very active outdoors. However, my D3 is low in the normal range if that makes sense. If you look at normal ranges I am the lowest point in that range. How that can happen with all the sun exposure is beyond me, but it’s a fact. Since being Dx’d with Prostate Cancer in 07, I have taken a 2000iu D3 supplement daily.