A 16 week old cockerel can defiantly fertilize eggs. If your question is how old do they need to be to produce offspring then the answer is a lot younger than you think. If your question is what age do well established breeding programs bring new cockerels in at, then it is going to be a lot older than 16 weeks. Some breeding programs won't bring in a male for breeding until after its first adult molt (i.e. 18+ months). There are lots of reason to wait until the cockerels are older. They are still filling at to 18+ months, Some color patterns are still developing until the adult molt is complete. There are some health issues that don't show up until the males is 18+ months old and if you don't wait you may be breeding them forwards, etc. I personally rotate cockerel on a 12 months clock.