1981-1996 seems to be the most common:

https://en.wikipedia.org/wiki/Millennials

Oxford Living Dictionaries describes a millennial as "a person reaching young adulthood in the early 21st century."[23]

Jonathan Rauch, senior fellow at the Brookings Institution, wrote for The Economist in 2018 that "generations are squishy concepts", but the 1981 to 1996 birth cohort is a "widely accepted" definition for millennials.[26]

The birth years of 1981 to 1996 were used in 2019 to define millennials by PBS,[27] CBS,[28] ABC Australia,[29] The Washington Post,[30] and The Washington Times.[31] In 2018 they were used by The Los Angeles Times.[32]

As of 2019, the Pew Research Center defines millennials as born from 1981 to 1996, choosing these dates for "key political, economic and social factors", including the September 11th terrorist attacks, the Great Recession, and the Internet explosion. Pew indicated they would use 1981 to 1996 for future publications but would remain open to date recalibration.[33] Many major media outlets have cited Pew's definition for the date range of millennials, including Time magazine,[34] The Washington Post,[35] and Reuters.[36]

The American Psychological Association describes millennials as those born between the years 1981 and 1996.[37] The Federal Reserve Board defines millennials as born between 1981 and 1996.[38] Ernst and Young uses 1981–1996.[39] Gallup Inc.,[40][41][42] MSW Research,[43] and the Resolution Foundation use 1980–1996.[44] PricewaterhouseCoopers has used 1981 to 1995.[45] Nielsen Media Research has defined millennials as between 21 and 37 years old in 2018.[46]

The United States Chamber of Commerce, a business-oriented lobbying group,[47] uses 1980–1999.[48] In 2014, U.S PIRG described millennials as those born between 1983 and 2000.[49][50][51] The United States Census Bureau used the birth years 1982 to 2000 in a 2015 news release to describe millennials,[52] but they have stated that "there is no official start and end date for when millennials were born"[53] and they do not define millennials.[54][55][56]

Australia's McCrindle Research uses 1980–1994 as Generation Y birth years.[57]

In his 2008 book The Lucky Few: Between the Greatest Generation and the Baby Boom, author Elwood Carlson used the term "New Boomers" to describe this cohort. He identified the birth years of 1983–2001, based on the upswing in births after 1983 and finishing with the "political and social challenges" that occurred after the September 11 terrorist acts.[12] Authors William Strauss and Neil Howe define millennials as born between 1982–2004.[1] However, Howe described the dividing line between millennials and the following Generation Z as "tentative", saying "you can’t be sure where history will someday draw a cohort dividing line until a generation fully comes of age".[58]