Why were slaves normally children and young fit men and woman?

Children and young fit men and woman were used as slaves because then they could work for longer and more than old people. Mostly ship people used slaves to load cargo and pick it up from other ships. If slaves didnt do as they were asked, they would get wipped and beated and badly treated for refusing to do work they didnt want to do. If ship people got a edely man or woman, they wouldnt be able to do as much as yonger people. This wasnt fair because they didnt even get paid for anything...