When Lance Armstrong tweets a link to an article, it's usually worth taking a look-see.
The idea that exercise is good for you isn't exactly news, but a series of independently conducted studies point to more evidence that exercise may help prevent the onset of chronic diseases such as heart disease, dementia and osteoarthritis.
One of the most interesting studies in the article was a Canadian study which looked at the cognitive effects of weight training (free weights, machines, squats) versus toning and balance exercises. The women, aged 65 to 75, who did weight training saw more improvement in memory and learning than those who did the toning and balance workout.
One possible reason is that with the resistance training, there were more things to learn -- how to use machines correctly or how to keep the correct form -- and there more things to retain -- machine settings and repetition counts. The physical benefits of the exercise were coupled with the engagement of the brain.
While more studies need to be done to replicate the results (and all studies have some flaw in their design or execution), the evidence is encouraging. In particular, the takeaway message is that it really is never too late to start exercising.