The efficency isn't linked to the age, exactly.
The %s are added not depending on the age, but depending on the own contributive roles assigned inside the team.
Basically, and Ive experienced many times this situation, if you actually have, many contributive players and the Roles of contribution are covered, you will not find, a new player, that performs well, cause the contribution roles are limited.
You can not have 5 key players scoring 40 goals each, basically, so the roles are assigned, depending on the own team status, and at some point to allow a new "excellent" key player to join your team, first, you have to create the space for him.
Thats why a super player that comes from another team via negotiations can be vanished if we sign them, the good performance don't belong to the player itself, the engine will only assign a power inrelation with what he have arround when joins your squad, and this means that, his "power/performance" is limited by default.