Being older; seniority in age. Often refers to someone older within a group or organization.
山田さんはクラスで年長なので、責任ある役を任された。
Yamada is the oldest in the class, so he was assigned a responsible role.
年長の社員が新人の指導を行うことになった。
A senior employee will mentor the new hires.
年長の兄がいると、何かと頼りになる。
Having an older brother is helpful in many ways.
In kindergarten, the 'nenchō' year — the oldest class (usually the year before elementary school).
うちの子は来年小学校だから今年は年長になった。
My child is the oldest in kindergarten this year because he starts elementary school next year.
年長の子どもたちは運動会で年少の手伝いをした。
The oldest kindergarteners helped the younger children at the sports day.