It is not merely that people are old that makes them susceptible to Covid-19. It is other factors that correlate with age that make them more susceptible. One of those factors could very well be lack of vitamin d.
That is an entirely subjective measure that can only be determined by each individual/household rather than something that can be determined centrally and applied to everyone.
It shows the ability of humans to preserve and propagate animal species when there is an incentive to do so. No need to worry about cows going extinct.