The real question for FIRE is not how to get enough money to retire early. There are many ways to do that, especially if you start young. On the other hand, if you put this goal off too long, there is no safe way. So here the simple answer is: start relatively young!
The BIG question is what to do with all of those years. You may think you can lie on a beach sipping pinacoladas for 25 years, but I assure you, if you are human you cannot. Same for that cabin in the Maine woods. Boredom and suicidal thoughts will intrude. Humans are a bit like sheep dogs - they are made to work - to use their bodies and brains for some purpose. Relaxation is great and necessary to restore and renew. But it is not a human lifestyle. Same for leisure. The Greeks saw clearly that leisure was necessary for a human life. But what was that? To learn, to think, to create, to engage in the political life of the community.
Bottom line: you not only have to create a solid financial plan for FIRE, but also a solid creative life plan.
I retired 15 years ago at the more or less standard early retirement age of 62. I could have retired a decade earlier from a financial point of view, but there were certain advantages for me in staying in the workplace (as a professor, I didn't qualify for emeritus status until 62, and important benefits like access to the university library came with that status).
For the last 15 years I have become an expert on an important topic (active learning) and have been invited all over the world to give lectures and workshops. Becoming an expert: a master craftsman, an artist, a writer, sportsman, cake baker, designer - all of these are well within reach in a decade of concentrated effort. That is what concentrated leisure is for!
But like the financial plan, it is important to start the creative life plan reasonably early. If you want to be a writer, sports fisherman, artist, collector of whatever, you do not need endless leisure to get started. Take active steps and get a solid start at the same time that you start saving for retirement.