You can read about them on most Major Avian Research Journals. These are more of a guideline for when is best to teach certain birds behaviours and manners for life.
Eclectus & African Greys & Senegal & Amazon do not have this period. They learn at the relative same rate throughout their entire lives (allowing for high mimicry in time), you could compare this to how humans learn vs. a prolonged "Fledging" type of time.
Most other Parrot Species (e.g. Cockatiels, Conures, Love Birds, and Macaws) have a window (Between 6 Months to 5 Years) where they learn MUCH more rapidly. After its past they can still learn although it is much slower and they often return to old habit.