Vitamin D is either synthesized in the skin through exposure to ultraviolet B rays in sunlight or ingested as a dietary vitamin D.
The version made in the skin is referred to as vitamin d3 whereas the dietary form can either be D3 or a plant-origin molecule, D2. 
The 2 forms available through diet or supplements are Vitamin D2 (ergocalciferol) and vitamin D3 (cholecalciferol), both forms are well absorbed in the small intestine.
Neither ageing nor obesity alters vitamin-D absorption from the gut . However, older people are at high risk of vitamin D deficiency due to age and people with darker skin are less able to produce vitamin D from sunlight.
More than 90% of vitamin D for most people comes from exposure to sunlight