"Their 25 year old men against our 18 year old children", comparing the extremes rather than the typical.

The math is really quite easy:
1 - Look at the list of starters and how many of them have served 2-year missions
2 - Divide that number by the total number of starters
3 - Multiply the fraction in #2 above by 2 years and that is the number of years a typical BYU starter is older than their opponent.

This assumes we use gray and redshirts in the same proportion as other schools.

I've done this math a few times over the years and with somewhere between 35% to 60% of our starters being RMs in any given year, our starters are generally about 9 months to 14 months older than their typical opponent's starters.

BYU's non-LDS coaches over the years have remarked on the maturity of our guys' character, but have never expressed an opinion that we were at a physical or athletic advantage.