少年不知精可贵老来望比空流泪
[shăo nián bù zhī jīng kĕ guì lăo lái wàng bĭ kōng liú lèi]
Meaning 'In youth, one does not know how precious it is; in old age, they just regret in vain.' It refers to the realization that valuable opportunities or aspects of life (often associated with vigor and health) were wasted in youth, leaving regrets when one grows older.