同い年
おないどし · onaidoshi · JLPT N1 · noun
age
🎂 同 = same, 年 = year: 同い年 is short for 同じ年, so you were born in the same year.
Spoken Japanese shortens 同じ年 (same year) to 同い年. Use it for people born in the same calendar year, such as classmates.
同 — 冂 enclosure + 一 + 口 mouth
Originally depicted multiple mouths gathered under one cover or enclosure, suggesting people speaking together in agreement. From this image of voices joining, it came to mean 'same,' 'together,' or 'alike.' The shared cover symbolizes unity.
年 — grain 禾 over person 人 (oracle bone)
Originally a pictograph of a person carrying a bundle of harvested grain on their back, representing the yearly grain harvest. Because harvests came once a year, the character shifted from meaning 'harvest' to meaning 'year.' The modern form 年 obscures the original 禾 (grain) + 人 (person) structure.