Every year, the same argument and the same whiners emerge complaining that it isn't "fair" that the BYU guys are so old, etc. etc. Do you realize how many times this same argument has showed up on this message board? And how many team titles has this advantage given the BYU men's team? ZERO, absolutely ZERO.
Take a moment to consider the headache that the whole situation would make for a coach. So, for the typical coach you get an athlete for a solid 5 years- pretty much no one goes straight through without a red-shirt year, it just wouldn't make sense- why wouldn't you give it an extra year to see how the athlete develops? So, you get the runner as a freshman, they take basically a year to adjust to college, some of them don't even make the transition, they drop out, or they quit the team because they can't balance everything that college is throwing at them and they choose to focus on academics and a career. Great, so now you have the group that made it through the first year, they have started to build their mileage, put in some base and you finally have something that you can work with.
Now, consider that you will now have no contact with any of these athletes for the next two years- even if they kept running, how much of a nightmare would that be?
As for what missionaries actually do on a mission. Just look at the interview with Josh Rohatinsky about how much he ran on his mission.
http://www.runnersworld.com/article/0,7120,s6-243-292--11115-0,00.html
For the vast majority of the missionaries that is pretty much it: They don't have the time to do much more than maybe get out on their "preparation day" and play a game of soccer or touch football or ultimate frisbee, once a week, that's it. Yes, they have recently been encouraged to incorporate a small (<30min) of some sort of exercise (sit ups push ups, etc) in the morning- why? because the missionaries weren't doing any of it because they were too busy, and this lack of any exercise was becoming a liability to the church as missionaries got injured, gained weight, and got sick! (You can't get up early to work out by yourself [at least to go running, because you would have to drag your companion along with you] I tried to once- it involved running up and down the stairs to our basement apartment- all that got me was an injury because I couldn't run on anything flat when we went to play some football the one week and I almost blew out my knee because my legs felt like the ground should be 2 feet higher than it really was).
Or, consider the case of Josh McAdams, he came home ridiculously overwieght, and felt lucky that somehow he was still able to run- most of those that try to run afterwards don't end up so lucky. I spent a lot of time with the BYU team, although I was never good enough to be on it, and I remember when the current group (Batty, Bahr, etc) were freshman. There was always a long list of people who had great credentials, that, when they got back, either ended up injured, or just didn't have the desire, or could never regain the form that they had before their missions.
So, imagine being the coach and basically you have now have to go through the same process with the same guys all over again. And, to top it off, you start with those same guys, instead of them being in shape just coming off of a year of running they are either 20-50lbs overwieght from lack of activity or they are sick and 20 lbs under-weight from the parasite they caught in some third world country.
Yeah, that is just a coach's dream right there.
Don't ever tell me that the extra years gives anything other than an advantage in mental maturity, and hey, even then, that's not even a given- some people never grow up.