You don't "have to" weigh any particular weight. However medical people have created tables to show what are average weights for different people and what weights are considered healthy. These depend...
some people can weigh up to 1,600 pounds some are skinney and may weigh 70 or 60 of course a baby weights 2 to 22 pounds. Perhaps you should have worded it 'How much shoukd I weigh? my age is...and...