That's interesting. I recently ran a 5K which was composed of mostly teen age college students. At the starting line, I felt a little intimidated because I was by far the oldest runner in the field. However, as the race progressed, I noticed that their pacing was erratic compared to mine. I finished 19/120 overall with even splits, as I usually do.
I would guess that when you started running again in your 40s, you had prior running experience and knew pacing better than the teens who were trashing themselves in practice with little left for race day. Youthfull "irrational exuberance" exacts a toll on race day. Experience really helps and somewhat offsets our declining physical abilities. I'm 63.