This is something I think about a lot. I plan to never stop doing mentally stimulating activities, even in my old age. The difference you can see in cognitive ability between the elderly is staggering.

One particular instance that sticks in my mind was a video filmed in the UK where for some unrelated reason four older men and women were interviewed, with ages ranging between 80 and the low 100's. The mid 80 year old had trouble recalling information, spoke slowly, and generally seemed to exist in a fog so thick you could practically see it. On the other hand, the person around 100 was completely lucid, maybe even sharper than me! I can't say for sure that this is due to keeping busy and not partially genetic as well, but the variability really struck me.