Agreed. I've been very impressed by the infield bats on the farm thus far, and would like to see them get challenged more. I think June 1-15 area is a good timeframe to get them up (a month of ABs where they started). Really really excited for Gunnar at Aberdeen.
There will be rare exceptions (what they mean to the city, organization, generational type guy) but for the most part, I think getting rid of basically any player before they hit 30 is the way to go.
Sometimes, that means trading them at 27, sometimes it means 29. Situations and who the player is dictate all of those things but my basic rule of thumb would be to get rid of them by the time they hit 30.