Personally, I would say 9 months is about the best time to take a gun and go afield. It highly depends upon the dog. If the dog isn't interested yet, don't bother. If the dog is too immature (attention span), don't bother.
What I would do is when the pup starts showing interest, take them out to walk the fields alone, only the pup and you or with a friend but no real guns. The moment when she finds/chase a sparrow or such, fire a cap pistol. The pup should ignore it but if not, put it away then try it again the next day. Graduate to a starters pistol or .22 and use caps. A friend would be helpful here as he can stand perhaps 100 yards away and fire. Slowly let him come closer to shoot. By this time you should be using game birds. Some people skip the .410 but I'd use it anyways.
A gamefarm is great for teaching a dog. Alot of land and in a noise controlled environment. Use small birds at first like quail. Large birds can startle a small pup.
My dog got birdy at around 6 months. At 7 months, I introduced birds and the gun. The season started two months later. The first few hunts were walking hunts. The next few were asking other people without dogs if they want to hunt with her (never multiple hunters always just one gun). By mid- to late season we were hunting together.