you maybe able to tell sooner based on comb development but not the only thing I go by. at 13 weeks of age chickens start to get their "sex" feathers (saddle and hackle development) on cockerels these will be long slender and pointed...
my pullet
here she is older
again a little older
you will notice on the cockerels a color difference on the wings as the buff on the wings will become darker. which can be seen around...