MLB teams ideally have a mix of productive veterans, well-developed prospects and elite players in their prime years. That kind of overall age profile all but assures contention.
Each slide caption begins with individuals expected to spend most of the season on active rosters. They are categorized as either "young impact players" (25 years or under as of Apr. 1, Opening Day) or "old impact players" (35 years or above as of Apr. 1).
Clubs with more names in the latter group understandably didn't do well in these rankings (e.g. the New York Yankees). Athleticism, bat speed and velocity are all declining for players in their mid-30s. Despite the intangibles that those elder statesmen provide, they are often less valuable on the field and more susceptible to serious injuries.
Those with the "best" age profiles can sustain their level of performance throughout 2013 or even improve as the months pass without acquiring players from outside the organization.
*Asterisks indicate players expected to begin the regular season on the disabled list.