We buy-in the birds at 15 or 16 weeks old and normally they start laying within a couple of weeks. Occasionally we have raised our birds from day old, but the workload is a bit much with everything else we have to do and there is no cost saving.
The biggest single problem is vaccinations. Many of the vaccines are only available in large doses - 3000 or 4000. And if you are only vaccinating 300 birds the rest of the vaccine is wasted because you have to throw it away.