I started thinking about FIRE at age 24 (mid 1980s). I had a crude spreadsheet at the time that helped me conclude that liberal arts undergrad was not going to get it done. That led to business school (MBA) and Megacorp. I maintained various spreadsheets throughout my working career and always had a keen interest in investing and FI, and generally did a decent job of LBYM and staying on-track.
But, I would say that FIRE became a "priority" in early 2009 at age 48. Several things happened around this time... the economy was in shambles, work had become increasingly stressful, compensation was flattening, travel and workload increased, blood pressure was on the uptick, etc... but I also started to resent the fact that I had so little time for hobbies and other interests outside of work. I wanted to start a new chapter while I was young enough to keep the plot interesting.
So, I dusted off the old spreadsheets, added lots of new what-if scenario models, and started seriously looking for specific answers: When? What has to happen between now and then? The answers were more favorable than I thought. We were capable of retiring at that time, but the lifestyle "hit" would have been a little tough, and I was nervous with one in college, another about to start, and the highly uncertain economic condition at the time. I knew 55 was do-able, and that had always been the baseline plan. But given my work situation, it seemed like an eternity from age 48. So, we tightened the belt, doubled-down on savings, tweaked a few investments, and tracked the results continuously. With those actions and a cooperative market, I pulled the trigger at 52 in 2013.