you will rarely see a 20 year old rookie due to the draft eligibility rules
To be eligible for the draft, players must have been out of high school for at least three years and must have used up their college eligibility before the start of the next college football season. Underclassmen and players who have graduated before using all their college eligibility may request the league’s approval to enter the draft early.
Players are draft-eligible only in the year after the end of their college eligibility.
taken from:
http://operations.nfl.com/the-players/the-nfl-draft/the-rules-of-the-draft/so to be eligible at 20 a player would either:
A. have to graduate a 4 year college by age 20 or
B. have started college at 17 and played every year to use up their college eligibility by age 20(since most college leagues only allow 3 years of eligibility)
i believe that's why JDB uses 22 as the age for the draft (they are generated at 21 in week 16 then amazingly all have the same birthday and turn 22 at then end of season sim)