Is this age-related decline in GFR or kidney disease?

Yes that is true. While the physician's assistant may be technically correct to jump to ckd, it might be premature. And it might turn out to be so.
I feel like it could go either way and that my doctor is being careful to not jump too fast! We will see.,

@mimi234 I want to qualify my previous post and say that a sudden drop should be investigated. And of course your doctor can monitor you and explain things.

Technically an eGFR under 60 is called "stage 3 kidney disease" but my primary care doctor never even mentioned it (I saw it in my portal) and I did not get referred to a nephrologist until I suddenly dropped to 36. I think a lot of us "over 29" have eGFR's in the 50's.

I don't hydrate enough!

The question might be to consider what does a diagnosis change in terms of how you go on with your life? You could have a doctor who told you that you had an early and mild stage of CKD. If you heard that from your doctor, how would that change your life? I would say that I would probably want to eat a careful and nutritionally sound diet in either case. It sounds like you are not having any symptoms that require medication treatment so there would be no reason to start you on anything you aren't already taking, even if he called it early stage CKD. I would pay close attention to my labs to see if there is anything that is borderline (particularly minerals like potassium). Likewise monitor you blood pressure frequently. Other than that, why not just work with your doctor and see where this goes? What difference does it make if we call it age related kidney decline or CKD? How does it change your life either way?

Oh yes, my labs are good. Creatinine is 1.1 which is in the normal range. All others such as potassium are in the normal range.

Just wondering if your gfr has improved or if you are on a treatment plan?

I have HBP, Diabetes and possibly Neuropathy ( negative on the EMG test) now with one Kidney. I don’t have issues with getting appointments, just trying to keep everything controlled per my Nephrologist.

Also, another thought.
It seems that it might be important to know, for example, if the kidney is actually diseased, or if it is not. (If that distinction can even be made).
This can help with the prognosis, I assume.
I plan to work with my doctor, there is nothing else to do for the time being.

I guess if you really need to know to calm yourself you could try to get a kidney biopsy, but that seems a little extreme to my thinking. I was in end stage CKD for 10 years and was never given a biopsy.

That seems extreme. A sonogram might be useful.
This all started when my doctor's Physician Assistant was going over all of my stuff with me.
She said, ''Oh, your colonoscopy is great, your bone scan is very good, your echocardiogram is fine and your mammogram is great. But you have Chronic Kidney Disease and you must get a sonogram right away."
I called my doctor next afternoon and he said . You do not have ckd. You are fine. Come back in three months and we will retest.
I am not even sure if it is possible to discern the difference...
How did you survive being in end stage for 10 years??

@mimi1234 I had an eGFR in the 50's for years and it was not an issue. I think it is unfortunate that they call it "kidney disease" and even worse, "stage 3." Your creatinine is normal so wondering why your eGFR is 52 (which isn't too bad according to my doctors when mine is at that level).

With eGFR's in the 30's and 40's I do not have any treatment, no. I see the nephrologist once a year. My Cystatin C's are higher and the nephrologist said the truth is somewhere in the middle. I avoid NSAIS's and other meds or supplements that affect the kidneys.

I have lupus and high antibodies for scleroderma so there was some concern about an autoimmune attack on my kidneys since there was a sudden drop, but things are stable. My heart function was checked because that can be relevant to filtration rate, as well as low blood pressure.

Because I have low blood pressure the diet suggested for me is a lot of protein and enough salt, which is very different from most people with kidney disease. I believe muscle loss is another factor for me and protein helps with that.