Youth is not a time of life; it is a state of mind; it is not a matter of rosy cheeks, red lips and supple knees; it is a matter of the will, quality of the imagination, a vigor of the emotions; it is the freshness of the deep springs of life.

This quote is saying that being young is not only related to physical appearance, as some may think. It is rather a state of mind, meaning that it reflects attributed such as having motivation and determination, imagination, enthusiasm and energy. It is also a sign of a life full of new experiences, full of motivation and awakening.