You bring up two interesting points.
1) There is variation within breeds. You are absolutely right! If we look at individuals within a breed we see different pups progressing towards the end of the critical period at different rates. Differences in experiences during the critical period can further exacerbate later behavioral differences. Even littermates growing up in the same place can have different early experiences. But if we look at a group of labradors for example and compare them to a group of German shepherds, we see that on average labradors are progressing to the end of their critical period of socialization more slowly than German shepherds.
2) Something is going on in adolescence/early adulthood. The critical period we are focused on presently is the primary critical period of socialization, which happens in the first months of life. However, there has long been talk of the possibility of a secondary critical period of socialization toward the end of adolescence/ beginning of adulthood. Such a period has been found in mice, and while behavior is not as flexible during this period as it is during the primary critical period, it is still important in the development of adult behavior in mice. We currently do not know what or if there is an equivalent period in dogs, but I think it is likely and have long been interested in studying this hypothesis.