Nobody actually cares how old an NCAA athlete is. The people making a big deal about BYU athletes age are just finding something to hate them for. If they actually cared, they would get upset about the foreign athletes who are way older than other college athletes too. For example, Alabama's roster right now has Eliud Kipsang who is listed as being born in 1996, but is only a freshman. So he's nearly 25 already as a freshman... What about Vincent Kiprop? He was born in 1995 and is currently 26 and still competing.
What about athletes like Cheserek? It seems like he was a lot older than he claimed to be. He was one of the greatest high schooler runners of all time and most would argue that he was the best NCAA distance athlete ever. But after graduating, he ran a pretty fast mile and never ran anything comparable ever again. Almost sounds like he's a lot older and past his peak than what his "official" age says.
Realistically, it doesn't matter how old a college athlete is. College competition shouldn't be reserved for a certain age group.
As for the people asking why BYU athletes can't go on a mission after they graduate, most of the time they can't. By the time they graduate, most of them are married. Even if they aren't, no employer wants to hire somebody who got a degree and hasn't had a single job in the 2 years since. For high level athletes like Conner Mantz, nobody will offer a pro contract to somebody that plans on taking 2 years off of competing and serious training. Waiting to go on a mission until after college is just a really bad idea for these guys.
But again, it doesn't matter because college competition isn't just a certain age group.