I've noticed people tend to consider only 18/19 year olds as "trainees", is a 20+ year old really pretty much "settled" and not worth pushing in any stat?
Currently, due to the fact BB is a young game, most of the players are around the same skill levels. To more faster create a wider range of players the BB's sped up training for young players to double the normal rate. Over time the training rate for young players will be brought down to 1.5x the normal rate(young players still train faster obviously) . Though the BB's never said what age was considered "young", its assumed its 18-21ish. 18-19 are considered trainees because they give you the longest possible time to train them up.