Tā cháng shuō xuéwúzhǐjìng, suǒyǐ bāshí suì hái zài dúshū.
He often says learning has no limit, which is why he's still studying at eighty.
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From Liu Kai 刘开's Qing-dynasty essay 《问说》: 理无专在,而学无止境也 — 'truth has no fixed home, and learning has no end.' Often paired with 活到老,学到老 (live till old, learn till old) in modern usage.