I was almost 60YOA when I donated. I started with a GFR of 90. I was 60GFR at my first 6 month check up, then started dropping until I was 42GFR at 1 1/2 year and the creatinine was creeping up as well. I am now back up to 59GFR at 62YOA with the creatinine at under 1.
I would like to add that diet is huge after donation. Less red meat, including pork,maybe one meal a week, add a lot of seafood and fish, chicken moderately consumed. Less dairy and animal protein in general. I try to have one non-animal protein meal a day and that includes dairy. Add nuts of most varieties instead for a good protein substitute. Eat all the veggies you want, many are good protein sources and easier for the kidney to filter and a fair amount of fruit. Fresh or steamed is the best, frozen almost as good, stay away from metal canned foods in general, BPH in the can coating. All this I learned when trying to figure out what to eat. Organic is good if you are going to eat the outside of the fresh food and wild caught fish is best.
Drink at least 2 qts or liters of filtered water a day, but not a lot more in liquid or you will flush needed minerals out of your body.
If you are fairly physically active that can affect you test results as well. Creatinine is dead protein in the blood and that can be depleated muscle cells that your body is flushing as waste not just what you have consumed in food.
I was told by a dietician to keep my protein intake to about 35-40 grams a day for my height and weight. 1 oz of meat, or 1 egg is about 7 grams of protein. I would recommend making an appointment with a dietician and then keeping a food journal for about a week before so they can see what you consume in an average week. That is what I did and the dietician immedatly saw I was consuming more protein that I should be. I changed my diet and my labs where perfect at my last follow up physical. The creatinine had been creeping up and the GFR down each time i went in so I knew it was time to get to the bottom of this.
You must be your own advocate on this, your health is your business. Don't trust others to do right by you, you have to get all the information you can for your health and well being. Your kidney needs to last you a very long time yet. Take care of it.