Some cockerels will start crowing as early as 4 weeks. Some hardly ever crow, depending on their breed and their place in the pecking order. The Cochins are usually not really much of crowers, but I am not sure of the BSLs. I would think by they time they reach about 3-4 months you can start hear some of them trying otherwise.
If you are trying to distinguish your boys from girls though, just post their pictures and I am sure people on here can help you figure it out. Get a nice side shot to show legs, combs, wattles and feathering.
Good luck