Can younger generations learn about preventive health, not just the elderly or infirmed, when their grandparents live with them?

It depends. It depends on the grandparents' knowledge of healthy eating. I see many seniors who are misinformed about what comprises a healthy diet and over rely on nutritional supplements. Age does not always guarantee wisdom and knowledge. If the senior is in good health, is not overweight, stays physically active, and takes few medications and supplements, they are probably a more reliable source.