Depends on the type of dog. If its a pointer wing training starts at a very young age (depending on the interest of the dog). I've had GSP pups start at 8 weeks others at 16-18 weeks old.
I had one around 15 years ago. Could not get it to group under 4-5 inches. It was a .308 (love that cartridge). Had a hard time selling it. Finally got a wonderful shooting Sako Finnwolf. Have not shot anything else since.
What Doc said. I had the half moon happen around seven years ago. Had trouble with my right eye for a while. Now I use a Burris with a very long eye relief.