If I had had a son/daughter in my twenties, he/she would be around the same age as you.
If I'd had my child while I was still in my twenties, he/she would be roughly the same age as you.
ーIf I had had a son/daughter in my twenties, he/she would be around the same age as you.
「もし２０代で息子/娘を生んでいたら、あなたと同じくらいの年齢だろう」
in one's twenties で「２０代で」
around the same age as you「あなたと同年代」
ーIf I'd had my child while I was still in my twenties, he/she would be roughly the same age as you.
「もし２０代のうちに子供を生んでいたら、あなたとだいたい同じ年くらいだろう」
while I was still in my twenties で「私がまだ２０代のうちに」
roughly the same age as you 「あなたとだいたい同じ年」
ご参考まで！
カナダ 
