Mine started crowing around four months but his half Pekin son started at 8 weeks. It really isn't a breed thing so much as an individual bird thing as to when they start. My other cockerel of a different breed started crowing at 13 weeks. Most cockerels start crowing anywhere between two and five months I would say.
Do any look different comb and wattle wise or are they all looking similar? Cockerels should have big, red combs and wattles by 12 weeks. Pullets should just be starting to pink up and sprout in those departments.