Lǎorénjiā suīrán niánjì dà le, yīrán shífēn kāngjiàn.
Although the elderly gentleman is advanced in years, he is still very healthy.
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More formal/literary than 健康 (jiànkāng). 康健 is a near-synonym with the characters reversed, and it's most often heard in well-wishes — birthdays, New Year greetings, letters to elders. In neutral everyday speech, default to 健康.