I'll tell you what.. I picked up Judah and his brother (for a friend) at the same time. At the time of 8 weeks they were separated. From that point on they were socialized differently, and trained differently. Judah hiked with me and many of my friend's adult dogs of all different breeds. Diesal on the other hand simply went to puppy classes for a few weeks. Judah experienced bikes, horses, a variety of people and vehicles, shopping carts, gunfire, fireworks, fire, drunk people, children, babies, screaming babies, other puppies, cats everything that I could condense into those 10 weeks that I had him before he hit 16/17 weeks...I introduced him to everything.
Diesal's socialization on the other hand just about ended outside of the other dog at their house, and the occasional bon fire with some friends.
Diesal gets spooked...very easily.
He lives in a household of Soldiers, yet is very nervous around gunfire and fireworks.
He freaks out when he sees a horse, and slams into people and children because he doesn't know how to interact... he's getting better... but Judah on the other hand just in the last week has met a ton of new dogs and people and has not had any issues - he's even exceptionally non-reactive around semi-reactive dogs.
Not saying my training was perfect. He still scoots across the house when we fluff up a plastic bag for the garbage can... and he gets nervous your typical upright vacuum because i simply didnt have one when he was young. But being able to compare brothers really verified with me how important those first 8-10 weeks that you have them in your possession is. I probably will never purchase a puppy from a breeder that keeps them till they're over 12 weeks simply because of my personal observations.