Youth = body and/or mind
The vitality and exuberance of youth can exist in the body with/without the corresponding mindset, and in the mind with/without the physical attributes.
Quotes to consider:
“Youth has no age.” – Pablo Picasso
“To get back to my youth I would do anything in the world, except take exercise, get up early, or be respectable” – Oscar Wilde
“There is a fountain of youth: It is your mind, your talents, the creativity you bring in your life and the lives of people you love” – Sophia Loren