Depending on your. Fitness level and any associated physical limitations, you can participate in P90X. Remember, it can be modulated for various fitness levels. But I'd stress that an older exerciser really needs to understand his/her limits, as there is literature to suggest that seniors are at higher risk of injury with these high intensity interval programs than younger persons.
P90x. Different seniors are at different levels of overall fitness. Do what you can and try to push your body in moderation! It will strengthen even as you age!
Possibly. They should be cleared medically by their physicians.