"In other words, a 22 year old has conceptually already had three years of "normal" development."
The keyword there is "normal" development.
If you've got very good training, medical and coaches, you might be able to get more development out of a 19 year old than his 22 year old counterpart. Conversely, if you suck at developing prospects (low medical, low training, ****** coaches, neglecting to optimize playing time, etc.), then you might stunt a 19 year old's growth such that it is less than his 22 year old counterpart.