Prospects should be below league average age. If you are league average age, you are old for a prospect. If he takes one step at a time from where he is now, he will be an old major league rookie at 25. At 21, you hope they are in high A. In Walker's case, he was a college guy drafted a year ago and starts older. Given his age and performance, the Twins probably should have moved him with Buxton. If he has a good first half in high A and moves to AA mid season next year, he will be on track. If he struggles in high A at 22, he probably isn't a good prospect.