After more consideration, I may push the age limit back up to 25 for next time. However, I am strong in my belief that there should be an age cap, despite the fact that some prospects may have a longer time line (eg Dickerson/Hanigan.) My response to guys like that is that they weren't really "prospects" in the traditional sense to begin with.
On the other hand, Stubbs and Heisey are obviously prospects. If either one is still toiling in the minors in 2011 (which would make them 26), I will no longer consider them to be "Top Prospects." Hence the age limit.
Most high schoolers get drafted when they are 18, giving them seven full seasons to make the majors before they begin losing some of their prospect luster. Most college players get drafted at age 21, giving them 4-5 years. While some players will surely take longer to develop, I think that's a fair time frame to give someone before you start questioning whether or not they still deserve "Top Prospect" status. Obviously, a guy like Humberto Valor will have nine years worth of mention- we'll surely be sick of hearing his name by then, at least in this forum.