Before this question can be answered, I think we need to nail down what is considered early and what is considered extremely early. But even that is not straight forward.
For early retirement, I would say that if you are younger than the earliest age when you can generally start collecting Government provided retirement benefits, you are considered early.
So for the American MMM forum members, before age 62 you count as early. 62 being the earliest age you can generally* start collecting SS benefits.
*If you start collecting due to disability, collecting deceased spouses benefits, ect, I think it's still considered early if you are younger than 62.
Other countries that have defined benefits have different ages for eligibility, so for that country, this figure could be different. I know nothing about the benefits system in Canada and Australia and I know we have a large number of members from those countries so they can chime in.
I think extremely early is actually easier to define as I think the best metric is someone who has retired X years before they can collect the Government provided retirement benefits. As we are labeling it 'Extreme' I think a good ground rule is 20 years earlier. So, based on the US SS age, if you are younger than 42, you are considered Extreme.
42 is also the answer to Life, the Universe, and Everything. Coincidence? Probably.
I think this is a pretty reasonable basis when one considers that typical American High School graduation is when you are 17-19 years old. If you immediately attend college and graduate with a 4 year degree, you are looking at 21-23 yrs old. That puts 42 at the halfway mark between when you leave school and the earliest most people would count on being able to retire. To retire before this generally implies a pretty significant devotion of time and energy to figuring out what you want to do with life beyond the typical, and working towards it with sustained energy, especially considering the debt loads many students have now.