How to predict income n years after graduation using income dataset with age, but not graduation year?
I think you're looking for false precision, here.  "Years since graduation" is already a step function with an essentially arbitrary cutoff (should it really matter to your model that much if somebody is one day shy of being at the end of +1 years, versus one day into +2 years?  the actual difference is a few days.)

Just use age minus 23, or just use age in your model.  It shouldn't impact things that much either way.