We asked a trainer to bring by his very well-behaved adult Schnauzer when our puppy Junior was about 10 weeks. That introduction went fine, although Junior was timid at first.
We also did a puppy class once a week at our local Petsmart store. He met six other dogs that way, plus random dogs every time we were in the store. That class started around 3 months. At four months, he went to dog daycare for his first half-day. Also at four months, his first trip to the off-leash park (with him on a long leash). So far, so good. Junior is pushy with other dogs, to the point where the adult dogs sometimes growl at him to get him to knock it off. But other than that typical Frenchie thing, he's very sociable.
Junior also meets a lot of people on his daily walks. We need to work on the jumping up, but he's friendly and doesn't chew kids' fingers or anything. I think it's also important that they hear traffic noise and sirens, see bicycles, skateboards, etc. Nothing seems to scare him.
Lisa