To keep the mind you had in your youth you should use it. Think of it as a muscle. Use it and it grows stronger. One of the major problems is atherosclerosis. Loss of blood supply to the brain by clogged arteries will mean less oxygen.
Less oxygen means less thinking ability.
See the cardiovascular disease page for suggestions on how to correct this problem. Gingko biloba can help considerably by increasing blood flow. Toxins and poor nutrition can contribute to neuronal damage. See the Basics of Health for information about detoxification and basic supplements. Flax Lignan Gold , Perilla Oil and Udo's Choice are rich sources of EFA's.