It's not like BYU has a special exemption no other school has. Even if we're talking about JUST the linemen. If you want the 18 year old in your system, GREAT. You're well within the NCAA rules to play him one year, send him off to GreenPeace for two years (where he volunteers to scrub the trees for a couple hours a day, and "anonymous" donors can make sure he's got a paid gym membership wherever he is in the world to spend the other 14 waking hours), and boom, you have a mature sophomore who can benchpress bulldozers.

Or, shoot, the clock doesn't start until they enroll in school. Why not start a lineman bootcamp where they're not in school, they just work out for 2 or 3 years, your boosters can even ACTUALLY pay them "for being on staff at the gym" without getting in trouble with the NCAA, and start them into your program at age 20 or 21. 5 years to play 4, boom, you too have 26 year old seniors.

Why does NOBODY do this??? Because it's insane. If there were ANY kind of advantage, SOMEBODY would have been crazy enough to try it out. To date, NOBODY has. Aww shucks.