Sharp eye Bruce, we finally, and I mean "finally!" were issued knee and elbow protection in 2005, there were two variations. The CADPAT green ones shown in the first photo man on the left, and off the shelf pieces by "Hatch" worn by the man on the right. I got a full set of Hatch (knee and elbow) issued to match the set I had been wearing for years. I never got the CADPAT ones for some reason, perhaps I declined to go to get issued those ones, cannot recall. There were varying opinions about which ones were best, but most guys, including me, found the CADPAT ones to be bulky and more restrictive.
In the second photo you will notice that Cpl. M (Still serving, a Sergeant now.) has only one and it is rotated to the ankle, this was a common practice, in an effort to reduce weight and bulk, only the dominant knee pad was worn, and when not in immediate use was rotated to the side and placed by the ankle, in the "travel position" as wearing such pads did cut off blood flow on long marches. The CADPAT pads could not be used in this way, they were constructed more like sports equipment, hockey gear comes to mind.
Almost no one wore the elbow pads during my time, they weren't worth the weight and bulk, and the majority of knee pad use was seen during tactical pauses when patrolling (take a knee and cover the arcs) or when firing from the kneeling position. Kneeling on rocky Afghan soil without pads, given our packed loads at times would feel like dropping a knee into a box of construction nails.
As any current or retired Infantry Soldier reading is nodding sagely, there is good reason why every Infantryman has ruined knees, feet, and back, and generally survives on Ibuprofen. The stuff you don't worry about when you are 18 sure does catch up when you get just a few years older.