首楞嚴義疏注經 2.20

2.20

王述無常念念遷謝，其理必然，故印如是。欲其更敘遷謝之相，以老少相比為問。十五曰童，未巾冠也。齡，年也。

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.