Have Dr. Cherry....er....Hyman show us one example of someone that age having the body and ability of a 40 year old. Tom Brady and Lebron both physically wore out like everyone else and never would have made it in their respective sports that long without the rules being changed. Getbiggers are exempt from aging though so you are safe.
There are examples in his book of very old people maintaining their health and fitness levels.
Tom Brady wasn't worn out. From his last season at age 45:
4,694 yards, 25 TDs, 9 INTs, Completion Percentage: 66.8% (with a league-leading 490 completions on 733 attempts), Passer Rating: 90.7. Rushing: 1 rushing touchdown with -1 total rushing yards on 29 carries.
That is amazing.
I can't stand LeBron, but as a lifelong Laker fan, I watched almost every Laker game last season. LeBron was hardly worn out. He still played at an elite level at age 41. Again, amazing.
But Hyman's book isn't really about elite athletes. It's just being functional at any age. He made a believer out of me.