www.drjensdogblog.com/socializing-your-puppy-why-later-is-too-late/
Additionally, I recently came across an AKC article the essentially reiterates the same and will share it as well. It isn't quite as in-depth as Dr Jens and doesn't discuss the critical periods in a pups life as well... but I think it's worth sharing as well. So another link to ponder.
www.akc.org/expert-advice/dog-breeding/how-to-socialize-a-puppy/?