Since all the physical reasons for slowing down have been covered let me add that there are other reasons for slowing down. First you just get tired of the same thing over and over again. The problems are the same, the non-solutions are the same, and the nagging by family members is the same. In addition to this you're treated as though you're an invalid and your brain has lost it's ability to function. Your skills are considered outdated and useless just as you are. Doctors start talking to you like you're a child who can't understand simple directions. And if you want to know about the over medicating of the elderly. Well, the more drugs that are prescribed the more of a kickback the doctors get and the more 'sedate' the elderly are kept the less family members have to participate in their lives. Yes, the body ages but that doesn't mean you're dead even if the younger members of our society would like us to be!
By the way, I just turned 70 and am working on my BS degree in Information Systems Technology. I have a 3.78 GPA and expect to graduate in 2018 when I'm 72, and that won't make me the oldest graduate.