Why do you get tired at the age of 78?

Well stuff is starting to wear out. Your oxygen transfer from your lungs to your blood is probably not as effecient as it once was. You may be developing a deficiency of some sort in your blood that may need geritol or something like that to help you out. If this came on you pretty quickly you should see a doctor to make sure you haven't had something go wrong in there. Hang in there. You're not done yet.