A person can weigh 150 pounds but that doesn't mean they are fat.
When it comes to weight you need to consider body composition. We need to remember that muscle weighs a lot more than fat. So a body builder can be 150 pounds and be a lot healthier and fit. While a young adult can be 150 pounds, but have access fat everywhere on the body. This is an unhealthy person.
So.. it is important to consider the amount of body fat as opposed to just a person's weight because body composition plays a bigger role in health than your weight.
Good Luck! :)