I’m not sure I agree. Muscle loss doesn’t happen simply *because* we age, but is largely correlated with the theory of use it or lose it.
Technically, for everyone except Benjamin Button, aging starts at birth. But when we “lose” an ability is also (largely) affected by when we stop using it.
For example, at 54 Jack LaLanne beat 21 year old Arnold Schwarzenegger in an informal contest. Schwarzenegger’s response was “Jack LaLanne’s an animal!” (reference)
Aging *can* include loss of muscle, brain, immune function, bone loss, etc” — but I think we’re too quick to chalk it up to “aging” and not quick enough to look at how we live on a day to day basis.