Refers to the quality or state of being young, often with a positive connotation of freshness, energy, or inexperience.
I'm amazed by his youthfulness.
She keeps her youthfulness regardless of her age.
What's the secret to staying young?
His performance was full of youthful energy.
That was a mistake made because of youthful inexperience.
After his illness, he suddenly seemed to lose his youthfulness.
老い is the noun for old age or aging, the direct opposite of 若さ, the quality of being young.
若者 refers to a young person themselves, while 若さ is the abstract quality of youthfulness that a person can possess.
Derived from the adjective 若い (わかい, 'young') with the nominalizing suffix さ (-sa), which turns adjectives into abstract nouns expressing degree or quality.