Sleeping bag ratings confuse almost everyone, and for good reason: the headline number on the tag is usually the one you should ignore. Understanding the three figures behind it is the difference between a warm night and a long cold one spent regretting your purchase.
Reputable bags are tested to a standard that produces three temperatures. Comfort is the one that matters for most people — the temperature at which a cold sleeper can rest comfortably. Limit is where a warm sleeper is still just about fine but a cold one will not be. Extreme is a survival figure only, the point at which the bag stops you dying rather than keeping you comfortable, and it should never inform a purchase.
Buy on the comfort rating, and buy with margin. If you expect nights around five degrees, a bag with a comfort rating of zero gives you a buffer for the colder-than-forecast night that always eventually arrives. Children and older campers feel the cold more, so give them more margin still.
The bag is only half the system. All the insulation in the world does nothing underneath you, where your body weight crushes the filling flat — that is what a sleeping mat is for. A warm bag on a thin mat sleeps cold; a modest bag on a well-insulated mat sleeps warm. If the budget is tight, spend on the mat.
Small habits stretch a bag's range a long way. A hat, dry base layers, not going to bed cold, and a warm drink beforehand all add degrees for free. And never sleep in the clothes you sweated in all day — damp fabric against the skin will undo the best bag on the market.