Anyone born between 1981 and 1996 is a Millennial. Anyone born after is part of a new generation, whose name they haven’t decided on yet. (Informally, they’re Generation Z.)
Marketing firm Millennial Marketing defines a slightly broader, earlier window, stating that Millennials are born between 1977 and 2000. Meanwhile, Merriam-Webster defines the term rather vaguely as “a person born in the 1980s or 1990s.”
(I think tech usage is a key part of what a millennial is - I was 14 when I got my first computer and our house got an internet connection the following year IIRC.)
By your measure I’m a Gen-Xer but I think of them as people who were adults in the 80s - I was a kid.