Sleeping Bag Temperature Ratings
The idea behind sleeping bag temperature ratings is that a person will be comfortable sleeping in temperatures down to the sleeping bag's rating. However, these ratings are somewhat arbitrary because they are created by the manufacturer and because they don't take into account a) the person who is testing the bag or b) the size, shape, sex, and conditions of the person who is sleeping in it. One solution used by manufacturers for years is a copper mannequin. However, once again this doesn't take into account the person sleeping in the bag.
Sleeping bags that are similar with two different testers have been rated to completely different temperatures. Most outdoors people will agree that you should go with a bag rated at least 10 or 15 degrees lower than the lowest nighttime temperature you will encounter. It is always easier to unzip a sleeping bag and cool off than it is to get warm. So erring on the side of warmer is a good decision.
Sleeping Bag Specs
Other features of sleeping bags such as neck baffles, shape, length, and age will make a tremendous difference in the bag's true temperature rating. If a 5' person is sleeping in a 6' bag, there is an extra foot of space for that person's small body to warm up. It's best to buy a sleeping bag that is just a few inches longer than you are. Most manufacturers make sleeping bags specifically shaped for women (with a smaller shoulder area, bigger hip area, and shorter in length) and for kids (usually just shorter). It's good to keep in mind when purchasing a sleeping bag that after a bit of wear and tear, they lose some of their loft and therefore some of their warmth.