This interactive data viz shows the number of NHL defensemen who average 20 mins or more, per season, by age. On average, there are approximately 3 d-men per team who average 20+mins per game, therefor the data represents "top 3 defensemen" in the NHL. The bigger the circle, the more d-men represented by that group.
Let your eyes see the trend. It is clear that more older defensemen had a larger portion of the top ice-time minutes between 2006 and 2011. The ice-time has shifted toward younger defensemen in recent years.
However, the interesting visual is the diagonal distribution of large circles which represents a strong demographic group or draft cohort. Although there is a general shift of ice-time to younger groups in recent years, a few strong defensemen in one cohort can impact the overall shift greatly.
Take the example highlighted in the image to the right. It represents the "Karlsson cohort" which includes Josi, Gardiner, Brodie, Hedman and others who all play big-minutes on their teams.
Look at the circles diagonally; follow some of the other cohorts to see at which age the performance drops. Over time players drop out of the "20+ min club", reducing the impact of their cohort on the overall representation of impact d-men in the NHL. Sadly, no one can fight father time.
Interactive Chart (click circles to view players)
data source: hockey-reference.com