(T-V is a Lagrangian - it is not unique) This quantity is important in Lagrangian mechanics: see principle of least action.
... out with 17.4% under the age of 18, 3.5% from 18 to 24, 22.3% from 25 to 44, 28.7% from 45 to 64, and 28.1% who are 65 years of age or older. The median age is 50 years. ...