Vitamin D is either synthesised 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.