As we age our levels of B12 decrease as our gut function becomes less efficient. In fact this decrease actually begins at age 50 when the mucus membrane in the stomach begins to thin, resulting in decreased production of digestive enzymes and Hydrochloric acid (stomach acid). Stomach acid is essential for the absorption of B12.
B12 does seem to have a prophylactic action by supporting the immune system both directly and indirectly. Maintaining adequate levels of this vitamin form a crucial part of a healthy, well functioning immune system.