Wǒ fùqīn zhuàngnián shí néng kángqǐ liǎng dài dàmǐ.
In his prime, my father could lift two sacks of rice.
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The stages of life: 童年 (childhood), 少年 (early teens), 青年 (youth), 壮年 (prime, roughly the thirties and forties), 中年 (middle age), 老年 (old age). "In one's prime" is 正当壮年 or 正值壮年.
