At this point, they are based on the seasons. Rookie is for brand new players (age 0 at the start of the season), Prep is for players create the season before ( age 1 thru 40 at the beginning of the season), University for the players created 2 seasons before (age 41 thru 80 at the beginning of the season), etc..
As the season progresses, the age caps move also. Thus, on day 10 rookie leagues will be for age 0 thru age 10, prep will be for age 50 thru 90, etc...
There is no level cap, which means if you do not boost, you will lose several levels every season and be unable to compete against boosters within a few seasons.