Like humans, canines have biologically prepared critical periods for social development commonly called socialisation windows. Most of these windows of opportunity close by the time a puppy reaches 16-weeks of age, which is why correct socialisation at a young age is so crucial. This short timeframe shapes a dogs temperament- correct socialising will set your puppy up to be a confident, well-manned companion adult dog. Incorrect or lack of socialisation means you may have a puppy that grows up with a greater tendency to be fearful and potentially reactive.
Despite common folklore, you CAN teach an old dog, new tricks. But what you can't do is turn back time to this socialisation window and have a 'do-over'. Obedience behaviours are fun to teach, but correct socialisation is absolutely critical if you want to have a happy, well-rounded dog!