I had a period of time in my life when I worked at toxic place and my stress levels were so high I could not fall asleep at night. I would have constant flow of thoughts and was laying in bed for hours with sleep not coming. I could not leave that place for some time (circumstances out of my control). I found an article about a method some RAF pilots used during the war to fall asleep. Can't find that particular article I have gone through but google yields lots of results:
I tried that as last resort before going to GP but luckily this method worked for me. Later when I left that toxic place the sleep came back on its own.
I'd risk to say, at whatever age you choose to. On this very forum I have seen comments from 60+'s who were snowed under work and very happy (and not with 'old' tech), and also I have seen early 30's who were worried about them becoming too old for the industry. I believe it depends on your motivation, and maybe how you came into industry. Some are self-taught enthusiasts and some are formally trained workers, and some are in-between. Some are burned out by their circumstances, some are not. Some have family to feed and some are living life of a single. Superposition of all of this will result in mindset and how the one feels about their age related with what they do for living.
I'm in my early 40's and I think I will be doing what I do now until I retire (in whatever form it will be done, and if I'm not hit with brain desiese).
There was a time I was running a 10+ team and was executing scrum by the book, not really understanding much of it at start. Meetings felt weird and generally it was very intense time. But after some time a few things clicked in my head: I could plan with some certainty without bothering people, and execution was falling between min/max planned capacity, everyone was aware of pretty much everything, even in 'other places' where they were not actively contributing, everyone learned to estimate their work based on complexity. And above all, after leaving the place and working in a few different shops, when I look at the code quality produced in that place I'm still impressed (not brownnosing myself, really). We executed ~50 2-weekly sprints, I miss that time.
One man's side project is other's business, that's how I find mine. I have people in my network who are geared towards business, they know their niches. I build POCs. If it takes off I get %, otherwise I rip it off and opensource whatever makes sense.
https://www.google.com/search?client=firefox-b-m&sca_esv=a6b.....
I tried that as last resort before going to GP but luckily this method worked for me. Later when I left that toxic place the sleep came back on its own.