It depends on a lot of things; the dog, the owner, the land, time of year, etc. All dogs are different, some start early, some later. A decent owner would know when his dog was ready. I started one at 12 month and 1 day, she ran and killed her 1st hare without any problem; another, I waited unil she was nearer 18 month before entering her to hare. So a simple yes/no poll doesn't mean much.
Cheers.