Personally, I think 'Millennial' is more a state of mind than an actual age.
My wife is 31, paid off her college by working 60 hour weeks, and has had the same job for 10 years. No mortgage or debt.
Her best friend is 31, just bought a house last year and lives in the basement of the house because she feels safe there because it's where she stayed at in her parents house until she was 30. She makes $52,000 a year and is broke because she eats out at Applebees 5 nights a week and is behind on student loan debt because after getting her teaching degree she then went back for a degree in liberal arts.
Even though they are the same age my wife will punch anyone who calls her a millennial. Her friend will run to her safe space and focus on her breathing for 10 minutes if you confront her about it.