At a certain level - usually when he's over 75% full on his "Experience" meter - a driver's skills seem to start falling off, sometimes drastically, and he'll start making Zero-Bob mistakes and costing you races. At that point, it's time to retire him and start training a new Bob. I've retired several - some because they were past their prime, some because they just pissed me off at the wrong time by not listening. I'm a harsh taskmaster at times, it seems.