年長
ねんちょう · nenchou · JLPT N1 · na-adj
seniority
年 (year) + 長 (elder, long): more years counted than others = seniority
長 also means chief or leader, as in 社長 (company president). So 年長 literally means having more years than others, which is where the sense of seniority comes from.
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.