If saying 同{おな}い年{どし} in daily conversation is just something you'd say without thinking twice, then I have no further questions.
But, doesn't saying 同い年 sound sort of stand-offish. Saying 同い年 is a hint to the listener that he is misunderstanding our level of friendship.
My asking question to Empress Michiko at お茶会
(私): "その二人子、同い年ですか。
This is normal usage.
My sister-in-law asking me a question at お寿司屋さん
(義理姉さん response #1): "その二人子、年齢が一緒ですか。
(義理姉さん response #2): "その二人子、同い年ですか。
I'd expect response #1.
Hearing #2, I'd be like that was completely uncalled for. What is her problem? If she wants to keep her distance, then so be it.
BTW:
同い年 is considered 当て字, right? I've never seen a word made 当て字 by its 送りがな.