[holdenc]

More anecdotal...my mother-in-law is mid-70s. Retired for the last 20 years. Plays mahjong all day. Sharp as a tack, along with her mahjong friends.

[VladimirIvanov]

I plan to study and play chess and go once I retire. I also want to contribute to OSS. I think if you engage in truly intellectually challenging activities daily then you will be lucid for your entire life. The brain needs stimulation like a muscle in order to retain its ability over time.