I rarely discuss supplements in the context of longevity, as there is little to say: the weight of scientific evidence is overwhelmingly against most of what is sold under claims of providing some benefit to long-term health. Much of what is written out there in the world on this topic is written by people who sell supplements, and who therefore have every incentive to lie to you and lie to themselves in order to keep up a revenue stream. There are just a few exceptions to this state of affairs, where the state of evidence swings in the direction of benefits and few harms for ordinary people who are not vitamin deficient, and one of them is vitamin D.
Even here it isn't that there is an iron-clad case for taking it, just a decent case: there is good evidence for vitamin D levels in blood to correlate statistically with decreased mortality, there is very little sign of any harms from supplementing with vitamin D in animal and human studies, and vitamin D is very cheap. However it is magical thinking to assume that taking vitamin D to raise levels in blood will have the same effect as a natural variation observed in a population - one could imagine scenarios in which an artificially induced increase in vitamin D levels shuts off some useful process, for example, or in which natural variations in vitamin D levels are a side-effect of a beneficial process that is unaffected by supplementation. So there must still exist evidence to show that supplementing over the long term does actually produce benefits.
The rational person should spend all of five minutes thinking this through, make a decision, and then move on to much more important matters. The potential benefit here is not large in comparison to the potential outcomes of improved medical science over the next few decades, such as work on the SENS vision for rejuvenation biotechnology, so if we're going to spend time on thinking about longevity, far better for that time to go to SENS.
To investigate the association between serum 25-hydroxyvitamin D concentrations (25(OH)D) and mortality in a large consortium of cohort studies paying particular attention to potential age, sex, season, and country differences [we undertook a] meta-analysis of individual participant data of eight prospective cohort studies from Europe and the US [consisting of] 26,018 men and women aged 50-79 years
25(OH)D concentrations varied strongly by season (higher in summer), country (higher in US and northern Europe) and sex (higher in men), but no consistent trend with age was observed. During follow-up, 6695 study participants died, among whom 2624 died of cardiovascular diseases and 2227 died of cancer. For each cohort and analysis, 25(OH)D quintiles were defined with cohort and subgroup specific cut-off values. Comparing bottom versus top quintiles resulted in a pooled risk ratio of 1.57 for all-cause mortality. Risk ratios for cardiovascular mortality were similar in magnitude to that for all-cause mortality in subjects both with and without a history of cardiovascular disease at baseline. With respect to cancer mortality, an association was only observed among subjects with a history of cancer (risk ratio, 1.70). Analyses using all quintiles suggest curvilinear, inverse, dose-response curves for the aforementioned relationships. No strong age, sex, season, or country specific differences were detected. Heterogeneity was low in most meta-analyses.
[We conclude that] despite levels of 25(OH)D strongly varying with country, sex, and season, the association between 25(OH)D level and all-cause and cause-specific mortality was remarkably consistent. Results from a long term randomised controlled trial addressing longevity are being awaited before vitamin D supplementation can be recommended in most individuals with low 25(OH)D levels.