Here's a quick tip for navigating FA and deciding what players to extend/cut: position players' hitting and defense starts tapering off around Age 33/34, in particular their hitting power and defensive range (hitting splits can be maintained for several more years). So don't commit a long, expensive deal to a guy in this age range expecting him to continue to be able to play the same position and keep up his same hitting stats.
Pitchers, on the other hand, often do not decline until age 37/38 or even later.
Once a player starts showing a decline in a category due to age, that decline will only grow exponentially for the rest of his career.