Some of the best lessons in life can come from those who haven't been tainted by long stints doing the very thing they're commenting on.
It's not about age, but experience (and honestly representing yourself in the world).
N.B. I'm not saying he shouldn't write, or that he doesn't have valuable insights to offer, just that the author's experience is fair ground for discussion.
For instance I had done very limited management in my past. When a new project came up the company I worked at they had made me lead the project which essentially meant developer + manager. This employer also had the habit of under staffing teams while pushing very aggressive timelines. This lead me to try to manage a team of 6 while also coding around the clock for several months. When it came to review time I was a mess; I hadn't paid enough attention to my team, I was constantly scrambling for tasks, trying to find ways to help where necessary, etc. It was an incredible trial-by-fire.
When it came to my second stint at the same position for a new project I was ready. I gave constant feedback in real-time, I was able to keep on top of tasks, my meetings were more streamlined and I was able to estimate things far better; I did a 180 and became much more effective.