There are two main fear periods. The first happens at 8 weeks of age. The second happens at adolescence. Unlike the first fear period, that we know happens right around 8 weeks, the second fear period does not have an exact time frame. Perhaps its because dogs mature at different rates. Smaller breads tend to mature faster where as giant breads may not even enter adolescence until around 1 years of age. Or perhaps it's because this fear period maybe divided into many parts. Some trainers think that there could be a 3rd and even 4th fear period before a dog reaches full maturity.
Everyone does agree that fear periods happen when dogs become increasingly more confident and they are natures way of teaching dogs a healthy lesson in cautiousness. While we don't want dogs to be so confident they run straight off a cliff, we don't want them to become fearful of everyday things. One bad experience during a fear period can shape your dogs behavior for life.
For example, if your dog happily plays with an unknown dog and that dog suddenly has had enough, becomes agitated and delivers a bite with little warning, you now run the rest of your dog being scared of similar looking dogs or all dogs at that location. All memories for dogs are connected to a time, place and individual. During a fear period that memories amplifies and becomes very difficult to overcome.
Since we don't know when fear periods will happen during adolescence and we don't know how many fear periods your dog will have, treat any onset fear as if it's a fear period. At this time of life, we want to be sure that our dogs practice the best possible behaviors. We want our dogs to be confident, but under control. Redo a lot of your socialization at this stage in life. Take things slowly and don't force your dog into environments and situations that they are not comfortable with.