>I'm in my late 30s now, after two decades as an entrepreneur. My brain was quicker at 21, memory was slightly stronger (slightly faster, easier recall), and I had greater endurance at working productively in very long stretches.

I'm in my mid 30s. If my brain was quicker and my memory better when I was 21, it's certainly not enough of an effect to be noticeable. This corresponds to what more current studies say about when cognitive decline tends to become perceptible.

What has changed, is the demands on my time. Also, as you said, endurance. I can't sit at a computer for hours without stretching or something will start hurting.