Aging and decrepitude are minutiae of the game that are totally unnecessary but all follow a certain path of decrepitude.
Typically decriptude and age bring on a loss of strength, reactions, perception and reason. Intuition generally stays the same, as does charisma (in as much as the loss of reason can be countered by a winning charisma!) and vigor roughly stays the same (it is the last to go if it is high). Different species have differing impacts of decrepitude.