It actually is, have you ever notices the absolute void of developers over the age of 40? People yell age discrimination but it is not true, it is the constant churn in ones life. From constantly reeducating yourself to stay relevant to the next emergency that requires 16 hrs 7 days a week for 2 months. It requires a passion for technology just to fulfill the reeducation requirement not to mention the hours. It burns developers out. The worst part is those on the outside don't get it, they think we pay them all this money and they still whine complain and won't take our jobs. So we have a profession with a high reeducation to relevance cycle and extreme requirements of time. Those two alone knock out a good deal of the population from even entering the profession. Then you have to deal with the burn out once you are in the grinder which always take a few of the good ones every year.
So if you couple the education needs, burnout and with the fact that it is really hard to be a good developer you find that even the good one are out of the industry by 40. Usually through earning enough money to "drop out" start a worm farm in Texas. Or they hitched their star to the right wagon and have become very successful.
I know because I have seen it, as a 36 year old who has suffered two burnouts, I have seen every one of my contemporaries fall. I remain because I set my terms, I made a agreement to myself that I would get out of the corporate grind and work for myself.
I found a niche that was under-served (web accessibility, HCI for JS based web apps). So in a sense I am off the market, I don't think that I would go back to a corp gig for anything below 2x what the market is paying. But lets not fool ourselves as a freelancer, I am still for hire it just allows me to set some terms on how and when I work. It seems for me at least that this was enough to stave off a third bout of burn out. Being able to work from home and set my own hours has done a lot for my ability to refresh, recharge and reeducate.