They're still people with regular physiology, so I think it depends, just like asking if any other athletes improve beyond the age of 24. Jared Ward, of course, made progress in the marathon. (Which is probably less relevant since you'd expect people to get better with age). Back in the day, Doug Padilla ran his fastest times after college. Linkletter, who was mentioned previously in this thread, isn't relevant in this discussion since he didn't go on a mission. There are others/will be others from BYU that run post-collegiate that also didn't go on missions.
Recently, it seems a lot of the BYU guys go the marathon route instead of track after college. I don't know what mileage other college guys run, but Mantz seems to be on the high end of mileage, per his Strava. Assuming the other teammates are also running big miles, would you expect that to get them closer to their peak during college since they may not be significantly increasing mileage as pros? Anyone know of other current collegiate athlete examples that are training like Mantz...could be an interesting case study over the next few years.