Human inner strength does not increase with age as some martial arts novels describe; the older one gets, the more profound their inner strength becomes.
Inner strength is related to the body's essence, energy, and spirit.
Some people decline in their fifties; though they may still look well, their inner essence, energy, and spirit are like a nearly depleted gas canister, resulting in death upon contracting an illness, while others might appear frail but can still be hale and hearty, with keen ears and sharp eyes, nearing a hundred years old because they conserve their body's usage carefully.
However, no matter what, humans always struggle to overcome physiological aging, which brings more psychological issues. For this reason, in the theory of cultivation, thinking about cultivating only when you are old comes with more problems.