When a child or a teenager dies, people often think it's very tragic because they had their whole life in front of them. It wouldn't be as tragic if an old person died, because they had already lived a life. According to your argument, it would be more tragic if an old person died than if a young person died because the old person has already lived a life, which doesn't make any sense. If you were forced to choose between ending the life of an old person vs ending the life of a child, it would be more ethical to kill the old person who has already lived their life. An unborn child literally has their whole life in front of them, so killing them would be the least ethical thing you can do.