As previously stated no set age, it goes on feel!! Best to introduce it in a rabbit pen if you can get access, then you get to control the situation. Is it 100% on stop whistle even when there is distractions?
I've one cocker who is so hot he waasn't in the pen till 14months but another cocker I have will ONLY hunt when there is game about, so he was in pen a 4 months for a wee chase.
But I'll say it again, beg or borrow use of a rabbit pen, you won't regret it.