首楞嚴義疏注經 2.27

2.27

十年為限，麁相而觀。殂，往也。落，猶不住也。少壯不住，往而不遷也，故云殂落，流變易改也。

Taking ten years as the interval is contemplating coarse characteristics. 'Demise' (cu) means going. 'Decline' (luo) is like not abiding. Youth and strength do not abide; they go and do not halt; hence he speaks of 'demise and decline,' flowing, transforming, changing, and altering.