YOU SAID:

I didn't know who I was supposed to be at fifteen

INTO JAPANESE

私は15歳になるはずだったのか分からなかった

BACK INTO ENGLISH

I did not know if I was supposed to be 15 years old

INTO JAPANESE

私は15歳になるはずだったかどうか知りませんでした

BACK INTO ENGLISH

I did not know if I was supposed to be 15 years old