瑜伽論記 24.272

24.272

下中上坐者，毘尼母論云：從無夏至九夏是下座，十夏至十九名中座，二十夏至四十九夏名上座，五十夏已去一切沙門國王之所尊敬，是耆舊長老。

Regarding "junior, middling, and senior seated": the Vinayamātṛkā says: "From having no rains-retreat up to nine rains-retreats is a junior seat; ten rains-retreats up to nineteen is named a middling seat; twenty rains-retreats up to forty-nine rains-retreats is named a senior seat; from fifty rains-retreats onward, revered by all śramaṇas and kings, one is a senior elder."