I can't find the article anymore with all the recent "acquisitions" and subsequent site shutdowns. Basically, I remember reading that forwards peak at 23-25 while D-men peak at 26-29. I thought Hockey (was Puck) Prospectus once did a thing on this, but can't seem to find anything now.
D-men don't tend to fall into the steady state of decline that forwards have after their peak either. That conclusion may come from using GVT to compare players though. GVT needs much work for D-men IMO.
The big question is what you are looking at to define "peak" for D-men. Is it GF%-rel, 5v5 points/min, etc. Quantifying D-man value has always been uncharted territory. Until you properly define it, it will be tough to tell what age players peak with respect to your measuring stick.