Doesn't Kopitar still have a wrist injury? And we're talking about him exiting his prime?
Statistically, yes. NHL players prime years are 24 to 29. So this year he starts to fall into that category. Eric, who is a avid stats guy for Broad St HOckey, so good at his analytics, the Carolina Hurricanes hired him a few years ago to be a part of their coaching staff. And his studies, and analysis are a part of why they've drafted so well the last few years. He wrote quite a few articles on the subject of the rise of players, when the are at their best , putting up points, when they level off, etc. The first article was march 2012 and he went back to 1994 and used that data and has added to it since. In this article, he uses that to cite that players reach their points per minute played peaks at age 24.
http://www.sbnation.com/nhl/2014/3/13/5500522/nhl-scoring-stats-rates-age-analysis
Also (and the larger volume of this was in his earlier work, ) players are in their prime scoring ages 25 to 29 , with 18 -24 being their best points per minutes range
In his original article, he had few exceptions, Daniel Alfredsson was one of them. But it's a normal path the forwards take.
On average, players retain about 90% of their scoring through age 29, but the drop from there is pretty sharp.
This article uses similar data and has a chart , which shows that the points per minutes decrease after age 26 and you can see that in the graph at the bottom.
http://nerdhockey.com/by-the-numbers/2015/8/7/when-do-nhl-forwards-regress
And whatever the injury was (it appeared to be wrist to me when it happened but it might have been tendon in the arm) if it was /is torn and he's playing thru it, bad on the Kings, that is an area, that gets hacked and whacked on faceoffs an in corners and it will create more pain and discomfort. A lot of players choose to play thru rather than have surgery until the season ends and perhaps that is what he chose.
Or maybe the injury has healed and he's just having an off first half of the season.
But at 29, his prime years, where players produced at a higher points per minute rate , are behind him. He won't be putting up better numbers over the next 5 as he has over the last, that's just normal for his age.