I thought she was the same age as me ,but she turned out to be 15 years older than me with a kid.
I thought she was the same age as me ,but she turned out to be 15 years older than me and a mom!
こう言えますよ。
❶I thought she/he was the same age as me, but she/he turned out to be 15 years older than me with a kid!
I thought she was the same age as me,
(彼女のこと、私と同い年と思ってた、)
but she turned out to be 15 years older than me with a kid!
(でも実は 私より 15歳上で、子持ちだった)。
❷I thought she was the same age as me ,but she turned out to be 15 years older than me and a mom!
(彼女のこと、私と同い年と思ってた、でも実は 私より 15歳上で、母親だった！)
相手が男性なら、
I thought he was the same age as me ,but he turned out to be 15 years older than me and a dad!
(彼のこと、自分と同い年と思ってた、でも実は 自分より 15歳上で、父親だった！)
と言えますよ〜
参考になれば嬉しいです！
I have a friend who looks around the same age as me, so I assumed we were, but then I found out she's actually 15 years older than me and she has an eight year old daughter.
この場合、次のような言い方ができますよ。
ーI have a friend who looks around the same age as me, so I assumed we were, but then I found out she's actually 15 years older than me, and she has an eight year old daughter.
「私と同じ年だと思っていた友達が、実は15歳も年上で、しかも8歳の娘がいた」
to look around the same age as me で「私と同じ年くらいに見える」
ご参考まで！
アメリカ合衆国 
カナダ 
