noun
Archaic term; in modern Japanese, 同い年 (おないどし) or 同じ年 (おなじとし) is used instead. Found in historical or literary contexts.
古文書には「同庚の者」という表現が見られる。
In old documents, the expression 'person of the same age' can be found.
Compound of 同 (same) and 庚 (the seventh of the ten celestial stems, used in the traditional Chinese sexagenary cycle to denote years and ages). The exact historical derivation is uncertain, but it follows a pattern of using calendar signs for age-related terms.