Just looked at Bama and their avg is 22.7
The roster breakout is 124 players (39 Fr, 31 Soph, 31 Jr, and 23 Sr)
To get to an average age of 22.7 with the assumption of 100% red shirts (which is not the case), the average age of each freshman has to be 20.5 when they enroll.
After the redshirt year it would break out as
39 Fresh with an avg age 21.5
31 Soph with an avg age 22.5
31 Jr with an avg age 23.5
23 Sr with an avg age 24.5