A broody hen will collect a clutch of eggs before she starts incubating. Leave her to it – she naturally knows what she's doing! When using an incubator, it's better to get the eggs into the machine as soon as possible after they've been laid, having given them a chance to be "rested" for a few hours first. I've successfully incubated eggs when they were just over three weeks old, but that's very unusual. There's more information about how to choose the right eggs for incubation at this page.