Don't we wish that was true for all of us.
While genes and metabolism are important, age/hormones and lifestyle make a big difference. Obviously if you don't sleep enough, don't exercise, eat and drink rubbish then it will eventually show on your body.
And if you want to look as fit as some of the girls on this thread then they are clearly investing a lot of time to work out, eat healthy and look after themselves.
I am sure the ones that are successful also look after themselves (time out, exercise etc)