My Vit. D level was 19 the first time it was checked. It took a year to get it to 100 and I take 6,000 IU's a day, every day.
I am in my 60's. Once you get past a certain age, your skin no longer metabolizes the sun to create Vit. D. People who like in HOT areas, even tho the sun is bright, tend to spend more time indoors to avoid the heat and have lower levels. People of color tend to have low Vit. D. People with intestinal issues like IBS tend to have lower Vit. D levels.
Remember, if buying supplements or taking daily Vitamins, it's Vit. D 3 and not just Vit. D that you need!
Vitamin D has been proven to help extend the life of cancer patients and prevent recurrences. I've been taking it for 5 full years now.