It's a good idea to elect someone old as the pope. You don't WANT to have a Pope who's personal ambitions can steer the entire church away from centuries of doctrine. To use a captain metaphor, they want someone who will 'get the ship there', not decide to 'take the ship to Hawaii instead'.
Short popedoms mean that the church should, theoretically, never stray far from the course set in motion in the beginning. If it does, the idea is that it's not so far off course that it can't be corrected.
Everyone loved JP2, but his longevity was kind of awkward. He was a great guy, but... the age of New Pope is the result of some serious discussion behind closed doors regarding the danger of an eternapope.