In the flower of somebody's youth 的意思是某人正在风华正茂时。
Look at those students – they're all in the flower of their youth.
She's still extremely active considering she's no longer in the flower of her youth.
另一个和“花”有关的常用表达是 in flower，意思是“开花”。
Daffodils are in flower in early spring.
Do you know which plants are in flower in December?