Smoking may be bad for you, but I hardly think it causes dementia ... but certainly being engaged in the world with a sense of purpose (almost the key thing) and physically active seem to work very well, from what I've seen of older people in my family.  And the opposite occurs when an elder loses interest and purpose, as I've seen in the last couple of years with my mother.  She's nearing 85, and has 'retired' (her words) from more than just cooking the Christmas dinner.  And it seems her mind is retiring with her, her memory is failing, and while she's very comfortable in her home, she's become passive and indifferent to most things.  It's very sad, but at the same time it almost seemed to be her own choice to let go.  All we can do is keep her as comfortable and happy as possible.