At age 42 (11 years ago), going back to school and completing a Masters degree in Enterprise Architecture. I had been in the same job for 5 years when I made the decision. Work had a self-education program that paid for my tuition. The decision was literally made in the space of a couple of days before the mid-year intake. If I'd had to wait another 6 months, I probably would have talked myself out of it. It was hard work (had a young family at the time), but paid off handsomely when my stupid employer not only refused to utilise me in my chosen field (having graduated top of the class at their cost), but literally pushed me out of the company. One of my industry lecturers got me my first contracting gig and I haven't looked back since.
I guess I learned a few things:
1) don't overthink decisions (which is not to say "don't think");
2) to back myself and my abilities with the requisite effort. I'm typically smarter than I think but I need to put in a matching level of effort. When I got my Bachelor degree 20 years earlier, I literally skidded out the door in a haze of alcohol and with a shit grade. That cost me a few years;
3) don't be afraid of a challenge; don't be afraid of the unknown;
4) be sensitive to where you are in your life - can you afford to take a hit if things go pear shaped? Time-box your attempt to shake things up in your life;
5) If you work as a contractor - networking and self-brand management rules. I rely heavily on LinkedIn and the network of contacts I have cultivated, and keep my brand alive with posts and articles relevant to the kinds of work I want to be doing - not necessarily flavour of the month.
There's probably more but that's pretty much it. My income now is almost 3 times what it was in 2007, and while I'm not suggesting that's the only measure of success (far from it), it affords me a professional freedom to be more picky in the work I take on, and to live with far less fear than before.
EDITED TO ADD:
The reason I chose Enterprise Architecture was because it suited my temperament. I discovered I was a "systems" thinker pretty early on, and as I moved through a typical IT career trajectory, the "systems" I was thinking about became bigger and bigger. EA probably sounds pretty passe compared to all the "it" technologies people are playing with, but it's kinda like politics - reality is gritty, the problems are hard, endless and fascinating (if you're so inclined).