I clearly see both sides of the BYU/Age Cheat argument. I'm almost 60 years old now, and I last ran on a D1 Track/Cross Country scholarship in 1980, just before going on a mission for the LDS Church. A little more on my running background:
I began running just before track season in my sophomore year of high school. So I missed three high school seasons. In my first time trial against the top two distance runners on the high school team, I ran 4:58 in the mile and was from then on the number one runner on my time. I averaged about 20 miles per week, almost all intervals on the track, and I finished 7th at the Utah state meet, running 4:32. I ran completely injury-free that first season . However, my junior and senior years were plagued with injuries while still never running more than 40 miles a week, even during the off-seasons. I sat out my senior year of cross country with injury, was injured again during my senior year of track, but still finished 3rd at State in 4:23.
I was fortunate to get a D1 scholarship at the University of Utah (back then they had a cross country team, which they don't now because it was cancelled a number of years ago). The summer before that first (and only) collegiate cross country season, I upped my mileage to about 45 miles a week, with a 20-mile long run on a hilly course, which I typically cruised in 2 hours. (Yes, I know--I had no business running a 20-mile long run as almost 50 percent of my weekly mileage, but I was young and dumb with no real running guidance. In my first collegiate cross country race, I finished first on my team, beating all the upper classmen, as well as the Utah state high school cross country champion from the season before, who was now my U of U teammate. Promising start, but I injured my back stepping in a hole during my final sprint in that very race. I had to sit out awhile with that injury, and was never the same the rest of the season. However, I did get to run in the WAC Championship meet, which was won by 24 year old Suleiman Nyambui of Tanzania. Second place was Michael Musyoki, and his UTEP team, comprised of all older Kenyans, easily won the meet. Nyambui had just won a silver medal in the Olympics the summer before that meet, and Musyoki had finished 2nd to Henry Rono in the Commonwealth Games. And yes, I DID feel that it was unfair for me, an 18-year-old, to be competing against the best runners in the world in their prime mid-20s.
Right after that season, I was called on a church mission. I would report in April. However, after quitting college and giving up my scholarship, I had nothing to do but focus on running. I ran about 40 miles a week, listened to my body, and used a scaled-down schedule I had patterned loosely after Lydiard's schedules in his book Running the Lydiard Way. The DAY BEFORE I left for my church mission, I ran a 4:10 mile in a time trial all by myself. I really wanted to not go on that mission, but the family, church and self pressure were too great.
On my mission, I probably ran no more than 10 days, and never more than 3 miles. Seriously. As another poster mentioned, if your missionary companion won't go with you, you don't run. I know that Conner Mantz did run a lot more than I did on my mission, but not enough to stay in even good high school shape. He gained 30 pounds. I lost 10 pounds on my mission, all muscle.
After my mission, I came home in terrible shape, and unlike Mantz, no one was holding a scholarship for me, so there was no "red-shirting." Then I met a girl. In the LDS Church, young people were the encouraged to not delay marriage; in fact, returned missionaries were encouraged to "find an eternal companion." get married, and have kids whenever "God sends them." I got married 17 months after my mission, and a baby came along 14 months after we were married. I worked all day on my feet at Morton Thiokol (where the Space Shuttle booster engines and military flares were made), and running and competing seriously never returned .
To make this long post shorter, I DO know what it's like to compete against world class runners in their prime mid-20s as an 18 year old. No, it's not against the rules, but it certainly isn't fair. Look at the huge jump I made in the mile in less than a year, after one collegiate season: 4:23 down to 4:10. That was mostly physical maturity. 18 years old vs. 19 years old. That's why they have age limits in high school.
Yes, Mantz (and before him, Henry Marsh, Doug Padilla, Ed Eyestone, Conner Mantz, and others have a big physical maturity advantage (and a mental maturity advantage as well due to their missions and more life experience. But still, they had to retrain themselves after a couple years of little serious running. If you're complaining about Mantz now, as a 24 year old, I can just imagine what you're going to say if he runs his remaining year of eligibility at 25 years of age. You've got a point, but you can't discount that most of us returned Mormon missionaries never regain the promise we showed in high school. Life gets in the way, and priorities change. Those guys did it, though, and they deserve respect and praise for doing so.