There is a significant decline in muscle mass with aging. Building muscle mass when you’re young is important. You want to bring as much muscle tissue as possible to the aging battle.
For many, the atrophy of the skeletal muscles is first noticeable after 40 years of age and for almost all by 50 years of age.Between the ages of 50 and 80, we lose almost half the number of muscle fibers we had in our 20s.
Studying the muscle tissue of a normal population of men, 
the number of fibers in the large thigh muscle (vastus lateralis) decreased by 50%, from 600,000 fibers to roughly 320,000 fibers as they aged.
Even conditioned athletes are not immune to muscle loss.
The loss in the number of fibers within muscles appears immutable.
The magnitude of the loss in muscle mass can be ameliorated to some degree by hypertrophy of the fibers that remain.
The mean Cross Sectional Area (CSA) of the fast type 2 muscle fibers decreases, while slow type 1 fibers tend to maintain their CSA even in the elderly.
This type 2 fiber loss and type 1 retention causes us to remain better in performing aerobically rather than weight lifting.
A motor unit is a group of muscle fibers controlled by a single motor neuron that projects from a nerve cell in the spinal cord. Without stimulation from the nervous system, muscle fibers lose their ability to contract and the entire motor unit may atrophy and die.
The number of motor units we have are constant from 5 years to 50 years of age, but then they decrease linearly.
The mechanism for the loss in the number of motor units with aging is unknown.
The question is whether the nerve dies first or the muscle fibers die first? Right now, nobody knows.
What is known is that if you bring more strong muscle fiber into the process of aging you will fair much better.
So Coaches.....when your athletes strength train you should be right in the room with them.
For coaches as well as athletes it is important to ........GET STRONG.