English translation of 当年
⇒ 想当年,我们都很幼稚。(Xiǎng dāngnián, wǒmen dōu hěn yòuzhì.)
Thinking back to those days, we were all so naïve.
be in the prime of one's life
⇒ 三十多岁是男人正当年的时候。(Sānshí duō suì shì nánrén zhèng dāngnián de shíhòu.)
A man is in his prime when he's in his thirties.
Translation of 当年 from the Collins Chinese to English Dictionary