YOU SAID:

I didn't know that only male lions had manes until I was 10.

INTO JAPANESE

私が10歳になるまで、雄のライオンだけがたてがみを持っていることを知りませんでした。

BACK INTO ENGLISH

Until I was ten, I didn't know that only male lions had a mane.

INTO JAPANESE

10歳になるまで、たてがみを持っているのは雄のライオンだけだとは知りませんでした。

BACK INTO ENGLISH

Until I was ten, I didn't know that only male lions had a mane.