There also the matter of timing with regards to extensions and things like that. Those age 30, 31, and 32 years are huge in terms of aging curves, and having the ability to negotiate a player's first contract extension to cover them is a pretty big deal.
I have almost come to the conclusion that you should try to extend all your young players through those seasons just so you don't run into the Prince Fielder problem of guaranteeing too many years to a player based on the fact he's younger than most free agents.
Like Fielder is getting roughly 24 million a season to play through his age-36 season. But Milwaukee should have given him a reasonable extension after his first 3 very good years, instead of a weird stupid 2 year contract that still left one arbitration year hanging on the end of it. They paid him $34 million for his arbitration years, so say they offered him 7/$110 which would be 4 free agency years at $19 million per.
I think probably every player takes that heading into his age-25 season, Milwaukee would have gotten an additional 4 years of an MVP candidate 1B, and would still be out of their obligation to Fielder after his age-31 season, when he's likely to be very good still but would be an easier player to let go elsewhere given his age at that point and his body type.
The Nationals got themselves into a slightly negative position, for example, by signing Ryan Zimmerman to the initial 5/$45 million extension heading into his age-24 season, at which point he had already established a near-star level of play. They could easily have tacked another 3 years on at $14 million per year (what he's getting paid 2013-2018 anyways) and made it an 8/$87 million extension that ended after his age-31 season (2016).
I mean, how is an 8/$87 deal covering age 24 to 31 any riskier than a 6/$100 deal covering ages 29-34, signed 2 years early?