Rooks is 23 and totally within the rules of the NCAA. These automatic reactions when someone accomplishes something great from BYU, like run an American Collegiate record, win an NCAA title or anything else most people would be excited about is a "let's run favorite." This excuse is older than the BYU athletes themselves :-)
It's not a stretch to see that BYU recruits top runners out of high school, has a great team culture and coach. Whether they go on missions or not, they would still be a top tier program with occasional national champions.
I am amazed that if it is a true advantage, more teams don't encourage their athletes to go off for two years, within NCAA rules (and not train), so they can come back older. I would guess most people wouldn't take that risk. Until other teams try this and we see a pattern, I don't think there's an argument. I have never seen proof that BYU's success is tied to the age of their athletes. How many NCAA XC titles have the men's team won? Just 1?! So this big advantage must not be that big of an advantage...
If BYU's recruits weren't already top-tier high school guys (two of their current XC top 5 were NXN individual champions), I'd also say you were on to something. If anything, their success is either delayed or put at risk - like many who don't return to form after their time off.
If they have had national champion high school kids or World Championship team U18 guys come on board and win a natty, it's not a stretch they would be champions at some point whether they were 22 or 24 when they graduate. Making an excuse to nullify their accomplishments is just not fair.
I also think that if the NCAA saw this as an advantage, they would change the rule. The truth is that the limited success that BYU has, is not statistically significant and the times where people make excuses for great performances are more likely tied to the athlete's dedication and talent. Their accomplishments should be celebrated.