I was 59 1/2 with a BMI of 25, which was at the cut off BMI, when I donated to my niece who was 37 at the time. This was last year on March 10th. Of course a younger donor is better as they would have a higher GFR. Which is best for both, giving both a better life after surgery with only one kidney.
My niece has IgA neuropathy, this was her 2nd transplant. The first lasted her 9 years. Hopefully this one will last her at least that long. I'm no world class athelete, but have always been health concious for the most part. Guess that is why I was able to donate at my age. Now since donation it has become an every day part of my life. A very important part of my life.
Your center my just be looking out for the both of you as far as long term out comes. Actually, I find this very responsible on their part. You can seek out other donation centers of course. Does your brother plan to continue competing after donation? If he is unable to perform as previously will he be able to take that disappointment. Most donors can continue with life as previous, but a few do have some changes that prevents them from some previous activities and life styles.