I don't think you're ever going to get a large group of people to agree upon what the age cutoff is for "early" retirement. I'd agree that the age of 55 is definitely an early retirement...but it's not the threshold.
I think most people would consider themselves "retired early", if they simply retired sooner than planned. Although that opens up the possible scene of those who want to work as long as they possibly can. For instance, awhile back we had a secretary retire a few months before she turned 78. I think she had 39 years in the federal government, and was shooting to hit 40. But then she ran the numbers and realized it wasn't worth it to hash it out another year. Well, technically, she did retire earlier than planned. But I dunno if I'd consider just shy of 78 to be "early".
And, what about people who retire, to get a pension, but then go back to work part time? My maternal grandmother did that. She officially retired at the end of 1980, from the federal government, at the age of 56. But then, she went back to work, part time on call at a hospital, basically working as many or as few hours as she wanted. She did that until 70. I guess I'd call that span from 56-70 as "semi" retired, maybe? It definitely felt like retirement in a lot of ways to me, because I remember her being able to take off and go on a lot of vacations in that timeframe, so even if she wasn't fully retired, she still seemed to have the best of both worlds.
I googled "average retirement age", and age 62 came back as the median. Age 62 was also the most common age to retire. I also found a statistic, that 63.1% retired between the ages of 57 and 66. But again, this doesn't answer the question of whether they go back to work doing something different.
Anyway, I'd say that if you can retire sooner than the average, that would definitely qualify you as an early retiree. So, I'd say 62 might be a better threshold to use than 55. But even then, I'd be reluctant to call it anything official.
At one time, I wanted to retire at age 46. And, in light of the way things have gone financially, I probably could have. I'm 48 now, and in some ways I feel a little "late", because I missed my original goal. Now I'm pushing for age 50-51, depending on how my financial situation goes, as well as the j*b situation. I do feel that if I push back much further than that though, it'll definitely not feel early. My grandparents retired at 55 (Maternal Granddad), 56(or 70, depending on how you want to look at it, for Maternal Grandmom), 56 (Paternal Grandmom) and 60 (Paternal Granddad). So, if I'm still working by the time I'm 55, it definitely won't feel early anymore, at least not to me, because that'll put me in range of where my grandparents retired.