It is a lifestyle choice...not a moment in time.
As you get older....you realize that you made choices in the past (maybe not the best ones....in some cases). However you can influence the future.
Most chaps, it is a function of defining moments that cause the change.
In terms of the quality of nutrients is one answer. The amount of exercise. However sleep and recovery are greatly overlooked as determining the outcome.
When we are young and stupid with think we are invincible. With age, we would hope there would be some wisdom and moderation.
However unfortunately, as we age - we tend to be less mobile - ironically exercise improves and extends mobility (Both physical and mental).
Being committed to something you enjoy is part of the answer: whether it is walking outside or inside on a treadmill - the answer is mobility and not one size fits all...