Yeah, I'm only a couple years off from forty and I'm pretty sure my vision is not nearly fifty percent dimmer than twenty years ago. Certainly my glasses prescription has changed slightly, but that is more about redirecting the light.
Having 50% less light getting to the retina is not necessarily that it looks 50% dimmer subjectively.
E.g. it's not like having a dimmer for your room lights at 100% and 50%.
I guess it's more like a 1-stop difference in photography (where again, half as much light hits the film/sensor from one stop to the next).
Here's an example with half the 50% less light hitting the sensor between the two pictures. It's not as dramatic as you expect (except in the "depth of field", e.g. how focused stuff is):
