Before you can properly appreciate this tale of a much-needed retcon, it’s important to get a primer on both of these episodes.
The TNG episode “Unnatural Selection” has the Enterprise crew dealing with a virus that accelerates aging, much as Kirk had to deal with in the TOS episode “The Deadly Years.”
Meanwhile, the DS9 episode “Doctor Bashir, I Presume” is about Dr. Lewis Zimmerman sizing Dr. Bashir up as the model for the Federation’s latest holographic doctor, but this leads to the revelation that Bashir’s parents had him genetically augmented at a young age, a practice which is outlawed in the Federation.