Albert Barnes Commentary Job 38:21

"[Doubtless], thou knowest, for thou wast then born, And the number of thy days is great!" — Job 38:21 (ASV)

Knowest thou it, because thou wast then born? - This may either be a question, or it may be spoken ironically. According to the former mode of rendering it, it is the same as asking Job whether he had lived long enough to understand where the abode of light was, or whether he had an existence when it was created, and knew where its home was appointed.

According to the latter mode, it is keen sarcasm. “You must know all this, for you are so old. You have had an opportunity of observing all this, for you have lived through all these changes, and observed all the works of God.” This latter method of interpreting it is adopted by Umbreit, Herder, Noyes, Rosenmuller, and Wemyss.

The former, however, seems much better to accord with the connection, and with the dignity and character of the speaker.

It is not desirable to represent God as speaking in the language of irony and sarcasm unless the rules of interpretation imperatively demand it.