Being the same age; a person of the same age.
彼とは同い年で、学生のときからの友人だ。
He and I are the same age; we've been friends since our student days.
同い年の友達と旅行に行く予定だ。
I'm planning to travel with a friend who is the same age as me.
子どもたちは同い年だとすぐに仲良くなることが多い。
Children of the same age often become friends quickly.