Yeah, almost all people with Dementia are elderly. But don't underestimate the elderly. They walk, run, do push-ups, exercise bikes, other various exercise machines etc. Since people with Dementia are routine-oriented, they often still do their exercise routines. For example, my job used to be to walk with a 76 year old woman, two miles, twice a day. She walked at least 4 miles a day and sometimes 6. I am not discouraging exercise to prevent Dementia, but it is not guaranteed to work by any means.
Consider: Ametures built the ark, Professionals built the Titanic!