What does Yan Zhenqing mean by "black hair doesn't know how to learn early, but white hair regrets how to learn late"?

Creative background: Yan Zhenqing was a famous calligrapher in the Tang Dynasty. His "Yan Ti" regular script, together with Zhao Mengfu, Liu Gongquan and Ou Yangxun, is also called "the four masters of regular script". He lost his father at the age of 3, and his family came down. His mother Yin placed high hopes on him, implemented strict family education and personally supervised the school. Yan Zhenqing is also very studious and studies hard every day. This poem was written by Yan Zhenqing to encourage future generations.

"Persuasion" in "Persuasion" plays a leading role in the whole article. "Persuasion" is interpreted as "encouragement". In this poem "Encouraging Learning", the author encourages people to keep learning. Only in this way can they increase their knowledge, develop their talents and cultivate their noble character.

The first sentence was written early, which is an objective phenomenon, which leads to the second sentence: study hard and get up early. The second sentence is a supplement to the first sentence, expressing that young people should study day and night, and only by studying hard can they serve the country and make contributions.

The third and fourth sentences say that if you don't study hard when you are young, it's too late to study now when you are old. "Black hair" and "bald head" in the sentence are metonymic figures of speech, referring to youth and old age. Through the method of comparison, it is emphasized that reading and learning should be early, and it is not necessary to regret it when you are old.