Forty is the new thirty! Yes! We have finally learnt the art of pushing everything almost a decade back, almost. Career, marriage, babies, and the list goes on except possibly health! The human body still clocks you as 40 years no matter how hard you try to prove otherwise.
Even if you do the activities you did when you were 20, it is unlikely that the body is going to support you in the same way. The thought is a depressing one but it also provides food for thought.