Suppose you buy a car in 2010 and use it very rarely. In 2016 you decide to sell the car and take it to a dealership.
The mileage is unusually low for a car of its age. Yet the car is still 6
years old chronologically.
In terms of wear and tear the car is 6 months old.
Taijiquan is about spending your life energy (qi) frugally.
That way, as you get older, your wear and tear is unusually low for a
person of your age.