For me.. it's mostly a 2-d vs 3-d thing. I know that isn't 100% true. But consider
super mario, defender or pacman vs call of duty, destiny2, or outland.
As someone who didn't really grow up with gaming (but played a bunch of games later in life), I'm kind of partial to the 15 year rule. It's arbitrary and I like that. That gives adequate time for tech advancements between the generations and game systems that old don't tend to get much additional development.
(There are people still releasing new games on cartridges for the old systems, they're just not making a ton of money on it.)
So, that's
my preference and it's a definition used by others. After 15 years, you're not going to see any manufacturer support and there won't be any significant official game launches or the like. You're not going to go into a store and find new games outside of collector stores where they'll charge a lot of money for an old game or an old game that was preserved new in box.
That definition works for me, but it's a fuzzy topic and folks can pretty much use any definition they'd like.
As an aside, I stopped gaming pretty much immediately at one point.
Fallout and Fallout 2 were, at the time and in my opinion, the greatest games ever made. I still think they're the greatest I've experienced.
Then, they released Fallout Tactics...
It came with so many bugs that it wasn't playable and I'd spent money pre-ordering the game. (This would later be quite common, or so the journalists and gamers would tell me.)
I didn't wait around for that to happen a 2nd time. I don't stand for abuse and I considered that abusive. The game literally could not be won until they released a few patches and I had paid for working software even before it was released because I liked the franchise.
It wasn't until just recently that I decided to do some retro stuff. I'll have to set up a VM so that I can revisit Fallout 2. (I'm not going to pay for another copy/license.)