首楞嚴義疏注經 2.20

2.20

The king related that impermanence shifts and wanes thought after thought; this principle is inevitable, hence he approved it, saying, "So it is." Wishing him to further narrate the aspects of shifting and waning, he asked by comparing youth and old age. Fifteen is called a youth, not yet capped with a turban. "Age" means years.