The World Health Organization defines disability in part as a mismatch between the features of a person and the features of the environment in which they live. While you can't necessarily give a blind person sight or make an old person young again, you can adapt their environments so that that mismatch is less pronounced–or doesn't exist at all. This is where inclusive design comes in.
It's a myth deeply ingrained in our society that there's an average or "normal" person. The idea dates back to the 1800s, when the Belgian mathematician Adolphe Quetelet attempted to find the numerical average for a host of body measurements, like the chest circumference of soldiers, as well as the average stature, the average weight, the average age of marriage, and even the average of death for humans. This attempt to calculate how an ideal human looks and acts ended up codified in design as the "average user."
We don't think of glasses as medical assistive devices, but that's what they were–at least until the 1960s or so, when designers got their hands on them. Through the force of design, glasses become instruments of self-expression rather than stigmatized objects that connote that the wearer is different.