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. Structurally, three or four sentences are antithetical sentences, and "black hair" and "bald head" echo each other and set off each other, leaving a deep impression on readers.
Extended data:
Persuade Learning —— Yan Zhenqing, a poet in Tang Dynasty
Night light, five chickens,
It's time for men to read.
Black hair didn't know to study hard,
Bai Shoufang regrets studying late.
Midnight is the best time for boys to study every day. When I was a teenager, I only knew how to play and didn't know how to study hard. When I was old, I regretted why I didn't know how to study hard when I was young.