I'm 52 and we have an interesting situation - typically 3 guys in their early 50's and sons who are in their late teens/early 20's. I usually get out in front to enforce regular breaks, and them am followed by the young'uns, and then the two other older guys are much further back.
When we walk, it's an average of about 2.5 miles per hour, but we have to stop frequently to let the other two guys catch up. So, we tend to average 1.5 mph.
Once, when we were heading back to the trailhead, downhill, the young'uns and I were able to hit 4 mph.
At altitude we slowed to maybe 1 mph. To be fair, one of the older guys has a bum knee. On the other hand, he spent a lot more time last year hiking and doing mountain climbing and his speed has increased considerably.
I see a lot of variation, depending on injuries, altitude, slope, trail conditions. One time we nearly overshot our destination because one of the older guys didn't believe we were hiking as fast as we were (downhill on an easy trail).