If 100% sure 13 weeks, that is a boy who hasn't moulted into adult feathers. Keep an eye out for what kinds of feathers pop out of the pinfeathers that are soon to appear. Girls will get big red combs, but later on in development. If you said that was 18 weeks, I'd say girl, but not at 13 weeks. The whole 18 weeks to egg laying or x weeks to egg laying is generally the soonest you'll see eggs. With shortening days, don't be surprised if they wait till 20, 22, 24 weeks to give you a first egg.