YOU SAID:

old people were young people before

INTO JAPANESE

老人は以前は若者でした

BACK INTO ENGLISH

The old man used to be a young man

INTO JAPANESE

その老人はかつて若い人でした

BACK INTO ENGLISH

The old man was once a young man

INTO JAPANESE

その老人はかつては若い男だった

BACK INTO ENGLISH

The old man was once a young man