Age segmentation

If you are 50+ then for marketing purposes you will be slotted into an age group 50-60, 60-70 and 70+. Sometimes the groupings are based around the retirement age (ie 50-65 and 65+). What you can be certain of is that the 30 years of life, post 50, is unlikely to be segmented into more than three, possibly four groupings. Why is this?

The reason for raising this question is that AOL and Digital Marketing Services have surveyed children between the ages of two and seventeen to understand their level of Internet use and divided their findings into four groupings (2-5, 6-8, 9-12, 13-17). The results were surprising as was the differences between the age groups. For instance, children between the ages of 2-5 increased their access to the Internet, in the last 2 years, from 6% to 35%. Eighty-three percent of 13-17 year olds now have Internet access.

Why is it that the years 2-17 can be divided into four age groupings but at the other end of life the last 30 years is lucky to make three categories?

Is the reason that there is very little difference between the age categories post 50? If this is the case then it would make sense, but I have yet to see any research showing this to be true. Anybody have a view on this?