Wikipedia (abbreviated): Vitamin B12 is a water-soluble vitamin involved in the metabolism of every cell of the human body. It is particularly important in the normal functioning of the nervous system and in the maturation of developing red blood cells in the bone marrow.
Most omnivorous people in developed countries obtain enough vitamin B12 from consuming animal-sourced foods, including meat, fish, fowl, milk and eggs.
Since there are few non-animal sources of the vitamin, vegans are advised to consume a dietary supplement or fortified foods for B12 intake, or risk serious health consequences.
The Mayo Clinic says:
Because your body is capable of storing several years’ worth of vitamin B-12, deficiency is rare. However, if you follow a vegetarian or vegan diet, you might be prone to deficiency because plant foods don’t contain vitamin B-12.
Older adults and people with digestive tract conditions that affect absorption of nutrients also are susceptible to vitamin B-12 deficiency.
Left untreated, a vitamin B-12 deficiency can lead to anaemia, fatigue, muscle weakness, intestinal problems, nerve damage and mood disturbances.
For older people, it makes sense to take a supplement either in a multivitamin or separately.
Medical advice is that an average person needs to absorb 2.4 micrograms each day (that’s millionths of a gram, or on labels µg).
However, what you take into your mouth doesn’t all get absorbed, especially as you get older, so the recommendation is at least 10 micrograms each day. Most multivitamins will contain more than this, typically 25 micrograms. For Vegans, there’s no choice in the matter, as without a supplement you can end up deficient in vitamin B12, with serious consequences.