YOU SAID:

I went missing when I was one thousand years old, just because I’m not that old

INTO JAPANESE

私が千歳だったときに私は行方不明になった

BACK INTO ENGLISH

I was missing when I was 1,000 years old

INTO JAPANESE

私が1,000歳の時に行方不明になった

BACK INTO ENGLISH

I was missing when I was 1,000 years old