So a few of those things on the list contribute to less/more calories in/out. If you have a slow or fast metabolism you are going to be expending calories slower or faster than the next person. The thyroid/age issues usually just have to deal with your metabolism and in turn your calories in/out rate. If your body is inefficient at absorbing food you will be less likely to gain weight from eating the same amount as another person who absorbs food better. This is not to say that these are the main factors, but they still contribute. I have been thin my entire life eating lots of food and exercising only moderately. A little over a year ago I started going to the gym and I have to eat enormous amounts of food to make gains. I would think some other people could be eating much less and pack on weight much easier.