Below is their roster. Finding out athlete ages is a pretty big pain in the ass, but you can see a lot of the details in the "roster year" section, where you have a bunch of guys with 2-3 gaps in their time being rostered. If I recall correctly, NCAA only does multi-year eligibility allowances with military service and missionary trips and at BYU it's pretty much exclusively the latter.

Their 2018 team finished third, but you're right that there aren't a lot of those older guys on that varsity squad. The names I know are up there in BYU scorers are Clinger, Linkletter, and McMillan. Out of those, McMillan is the only one with the telltale two year gap, which should make him 23+ right now. That's not substantially older than your average NCAA athlete, but he should also have at least two years more eligiblity left, even if he's listed as a senior.

But again, you're right as to the current team. They apparently had an excellent 2016-2017 recruiting class (highlighted by Clinger but it was apparently also deep too - I'm just at the edge of my depth in NCAA XC fanship) and the excellent 2018 XC team was the result of those classes. If those guys all run four consecutive years, then it's a normal college cross country team. If they start taking two years off (like Clinger is now) and then return at ages 23-25, that's kind of weird.