The "engine age" should not affect climb rates and cruise performance at all. You would see different EGT and N2 for the same N1 in a differently "aged" engine, but the certified performance will not change.
If you find a regime where the plane does not perform as you would expect, I would ask you to specify that, including a screenshot with a stabilized climb, showing dataref output of:
weights
temperature
altitude
airspeed
engine thrust
so I can take a look at that.
Thanks, Jan