I kegged a 7 day old pale ale last night, yes, 7 days old. Proper pitch rates, well oxygenated, carefully controlled ferm temps (including a ramp-up after FG was stable, which only took 4-5 days). Tastes excellent, not "green" at all, of course, pales and IPAs are usually best fresh anyway, but no off-flavors.
I put it on 30psi, vented a bit, then shook in 15 second intervals, twice. This might be the fastest beer I've had, but I always let the beer tell me when it's done, and this one was done. There's no reason to leave it in the keg longer, plus I'm almost out of beer in kegs anway.