Puppies' ability to familiarize themselves with other animals lost upon maturity?

When our Bichon was around 3 months old, he was introduced to many other dogs as a matter of circumstance – some big (Irish Setter mix), some small like him (in-laws’ Yorkies).

Back then, our dog fearlessly made fast friends with other dogs. With the dogs he met back then, over a year ago, he is to this day friendly and will readily play and otherwise be engaging. However, with any new dogs, he acts really skittish – even if the new dog is a tiny puppy. He acts doubly scared of new big dogs.

What happens in a puppy’s development that allows them to make a lasting impression that other dogs --regardless of size – are “safe” playmates? When does this stage end typically?