I have been an entrepreneur for 17 years and am now “middle aged”. In my early years, I made many, many mistakes that could probably be pegged to youthful hubris. I also had no idea how to run a real business because I had scant experience even working for one let alone crafting anything approaching a “go to market plan”. If I were to start a new company today, I would probably lack the carefree creativity of a 23-ye…
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. And despite that, due to learning how to work more effectively, how to be far more productive with my use of time, understanding what mistakes to avoid, I'm a dramatically better entrepreneur…
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.