In the lower back discs undergo changes once they sustain damage. Following early injuries to the disc such as annular tears, people can develop disc prolapse, disc dehydration and subsequent narrowing over years and decades. When seeing patients and viewing thousands of MRI scans and x rays over three decades the pattern emerges quite clearly. Fully hydrated discs do not tend to narrow. Look at the MRI scan above, notice the bottom disc has no narrowing, very unlike the four above. I have included my own MRI san of my back that shows the top disc is within normal limits but all below this level are narrowed.
The only link between aging and disc narrowing is time. You age with time and it takes time to narrow a damaged disc. Impulse is mass x gravity x time and is the force that narrows discs. Narrowing depends on the impulse forces exerted on a damaged disc (this term is perfectly acceptable as we know the normal annulus/nucleus structure of a disc) over years and decades due to posture, occupation and recreation activities. Narrowed discs are not an aging process due to several reasons.
Disc narrowing occurs in some very young patients, often in their 20s. If disc narrowing is a consequence of aging, then old people should consistently have narrowed discs.
The pattern of disc narrowing is not consistent within any one spine regardless of age. In my scan there are four narrowed discs and one hydrated disc that is not narrowed. In my patients scan the lowest disc is within normal height and fully hydrated. If aging caused narrowed discs i cannot see a reason why only some discs are narrowed while others are not.
The sequence of disc narrowing can be seen on serial MRI scans of patients over decades where discs with a prolapse undergoes dehydration and subsequent narrowing while discs that have normal structure (intact annulus/nucleus and hydration) do not narrow.
Unless a clinician has been exposed to serial MRI scans over a few decades and correlated this with patients seen, and has seen the same patient over decades what I am writing would be foreign to the clinician.