This week, I read in the paper that having strong legs was a good indicator for staving off dementia an extra 5 years. One theory is that people who have strong legs are generally fitter, but another theory is that having strong legs ensures better blood circulation, so the brain gets more oxygen and nutrients.

Not being a medical specialist, I can't comment on either theory, but doing tai chi and/or chi kung (qigong) will help strengthen your legs, as both are done standing. Indeed one of the often-stated purposes of the tai chi / chi kung system is to promote good circulation! Regular practise will help you in both obvious ways (improving general health) and less obvious ways (improving circulation), so keep on practising!