There are several mechanisms that can impact the transmission of light through the various layers of the eye, but in general as we age the transmission of the eye will decrease significantly and intraocular scatter will increase due to "yellowing or clouding" of the various layers.

In fact the IES (Illumination Engineering Society) which publishes recommended light levels for various applications (indoor and outdoor) will generally recommend much higher light levels (sometimes as much as 2X) for spaces that are used by predominantly by elderly people (nursing home for example) to accommodate for the loss in transmission we experience as we age.

As long as you have had your eyes examined and there isn't anything abnormal causing the transmission loss, it's just something to be expected as we age.