A. Skin elasticity
This is one of the ways to see how well you are aging. If a 60 year old has more wrinkled and hanging skin than another 60 year old then he is clearly aging worse. Breath-holding was another decent option but that can be affected by so many things other than how well your aging, plus some people have asthma, or are born with bad lungs, or smoke, theres so many factors with bad lungs so that can't be reliable.
Hope this helped. Have a great day!