Firstly, try 70+ years, with the part that nobody talks about being how video blended with pre-existing arcade games with physical props to create video games.
Secondly, it’s down to the person, but generally it ties to shifts in technology, tastes, and popular trends in the gaming sub-culture. We define generations pretty rigidly, but I‘d say it’s more down to the time period than anything else. Systems like the Zeebo, Ouya, Neo Geo AES, and others are either freakishly under or overpowered for the generations they’re defined under, but they still get placed because of the timing of their release.
And please remember, to all those in the back who only follow Nintendo consoles, that the big N has been pretty late to the party on multiple occasions. That means the 16 bit generation begins in the late 80’s (1987) rather than the 90’s, and the fifth generation predates the N64 by three whole years because our good friend Marty decided to crash the party.
Alternatively, Nintendo has also been the one to start generations many times. And they also have the most successful example of having two systems in one generation line (Wii U and Switch are both considered 8th generation hardware). The story of games involves giving flowers to every corner of the industry, no matter how obscure.