What is the main idea of "seeing white hair in the mirror"

In his later years, the poet accidentally looked in the mirror and found himself with white hair and unfulfilled ambition. In the bright mirror, only my aging face and shadow cherish each other, expressing the poet's feelings of retaining time and nostalgia.

Tell us that many things in the world can be regained. Only when time is gone forever, should we cherish the good times now and not leave regrets for our lives.