Generations are really a blunt and rather useless tool IMO. Besides "only 90's kids will remember this" memes.
Philosophers, novelists, sociologists, anthropologists, and economists used to define, study, and critique generational divides before advertising became such a pervasive part of the modern economy
That eliminates the marketing effort to convince a consumer that they are part of a particular group. Getting individuals to self-identify themselves as members of various groups is a highly effective advertising delivery channel. Convincing you to use groupings as a convenient, time-saving shorthand (like branding is a convenient shortcut) gets you to buy more, more frequently.
Marketers and advertisers fear you insisting upon evaluating situations and people one-on-one, as they come up in your life, and using the marinating and revealing effects of time to judge value and character.
And they still do...
It's not really fixed as far as I can tell. Around the actual turn of the millennium, millennials were said to start in 1980. Now it's more commonly pegged at 1982.