老当益壮 Old but still vigorous
Explanation
这个成语指虽然年纪大了，但精力更加充沛，精神更加旺盛。
This idiom means that although one is old, one can still be vigorous and energetic.
Origin Story
汉朝名将马援，年老后仍然被皇帝征召去打仗，有人劝他年纪大了，应该在家休息了。马援却说：“大丈夫志在四方，怎么能因为老了就放弃呢？老当益壮，应该更加努力才是。” 于是他带领军队出征，取得了胜利。
Ma Yuan, a famous general of the Han Dynasty, was still called up by the emperor to fight in his old age. Someone advised him that he should rest at home because he was old. But Ma Yuan said: "A true man's ambition is to strive for greatness and glory, how can he give up because of old age? One should be even stronger in old age in order to achieve more." So he led his troops into battle and won.
Usage
用于赞扬老年人仍然精力充沛，能够继续为社会贡献力量。
It is used to praise elderly people who are still energetic and can continue to contribute to society.
Examples
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老骥伏枥，志在千里。老当益壮，宁移白首之心？
lǎo jì fú lì, zhì zài qiān lǐ. lǎo dāng yì zhuàng, níng yí bái shǒu zhī xīn?
Old horses still have the ambition to run thousands of miles. Even in old age, why should one change one's mind?
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他虽然年过半百，但老当益壮，依然奋战在工作一线。
tā suīrán nián guò bàn bǎi, dàn lǎo dāng yì zhuàng, yīrán fèn zhàn zài gōng zuò yī xiàn.
Although he is over fifty, he is still strong and works hard on the front line.
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虽然已经退休了，但老当益壮的爷爷依然精神抖擞，每天都去公园锻炼身体。
suīrán yǐ jù xiū le, dàn lǎo dāng yì zhuàng de yé yé yīrán jīng shén dǒu sǒu, měi tiān dōu qù gōng yuán duàn liàn shēn tǐ.
Although he is retired, the strong grandfather is still full of energy and goes to the park for exercise every day.