Mason, 51, exercises, has a healthy diet, and is not overweight. his body and organs are losing capacity. yet because he has not had any major bodily stress, his body will function well throughout middle age. this reserve energy that hides the loss of body and organ capacity is called:

organ reserve

Explanation:

Organ reserve refers to the functional capacity of our organs to support life.

Our organs have normally the capacity that it takes to simply function when we are young. Factors that stress those organs, like illness or injury test the organs, but because they have substantial reserve, don’t tax their limits.  

As we age, this reserve diminishes, and the organs become truly stressed by these same factors.

i believe the answer would be negative peer pressure