But the average age of NHL defensemen is continuing to drop. Perhaps you're right and those defenders raised on this side of the dead puck era will begin to skew the age higher instead, just hasn't happened yet. When defenders being to lose their first stride in their late 20's, they start to get pushed out, unlike in the pylons of the 90's.
Young defenseman are phasing out the fringe older guys who are barely holding onto a job anyways, not effective players like McDonagh unless you think falls off a cliff all of a sudden, even if he gradually declines he will be a solid player for awhile with his skating ability.
Just look at a guy like Coburn and how much we valued his play last season, he's 34 this year and I have no doubts he will secure another contract this off-season. The young guys are primarily offensive contributors the league still has a significant market in veteran stability just because of huge learning curve of playing defense in the NHL.
Last year there were 50 defenseman over the age of 30 who played 40+ games, this year are already 48 who have played 30+ games on the season so the league is actually pacing for more games out of older defenseman. 5 years ago this number was slightly higher at about 57~ so it's not like we're rapidly phasing out older d-men but the ones they cutting out are ones closer to 40 or have gotten too slow to stay in the league, a defenseman between 32-35 with good skating will be just fine.