Well, you can argue till your blue in the face over what defines a generation, what seperates generations, and so-on and so-on, etc.etc.etc... But that won't change a damn thing. What has historically been accepted by the media and public at-large, right or wrong, will trump any argument, even if you're 100% correct in making your case. And what IS historically accepted and established and followed by the aftermarket industry, enthusiast clubs, and enthusiast mags, is the breakdown 351CJ listed. Back in the 60's things were very different than today, cars commonly received SIGNIFICANT upgrades and changes and re-engineers practically every other year. In the 50's, this was almost an EVERY YEAR occurrance. For those reasons, the way people defined "a generation" back then was far more loosely defined than we do it today. Today, so much more money is invested in engineering and technology that it is totally impractical to change cars on a yearly basis as drastically and as frequently as we did 30-40 years ago. Instead, today Ford invests GOBS more money into the engineering of a car, and there is a ton more "content" that is expensive to develop compared to the comparitively primitive 1st gen cars. Then they stick with that basic design for much longer, because it takes alot longer to realize a return on that much up-front investment. So the very definition of a "generation" has changed over time, but that doesn't undo what's was already defined and put to bed years ago on the earlier cars. You can't rewrite history, you can't go the revisionist route. Just because technically the 302 motor was closer to 4.9 liters, doesn't mean that the world must discontinue using the term "5.0". So....
1965-1973 ..... 1st gen
1974-1978 ..... 2nd gen
1979-1993 ..... 3rd gen
1994-2004 ..... 4th gen
2005-up ........ 5th gen
The only real odd-duck in here are the 71-73's. The 71-73's were a significant departure from the original Falcon-derived Mustang platforms, moving to a newly engineered platform that shared alot of content with the 70-up Torino. Using today's standard of what defines a generation, the 71-73's would certainly be their own generation of cars. HOWEVER, back then the passing of a generation was based on far more sweeping changes. The 1st generation was really defined more by the first evolutionary cycle of the Mustang, almost like the lifespan of an animal. It was born light, limber and sporty. As the years passed it grew bigger, stronger, faster, more aggressive and mature. Then, in 72-73 the EPA got involved, gas prices were skyrocketing, and it got weaker, fatter, tired, and died. Then in 1974, an ENTIRELY new car was born, an economy car, a compact car, from scratch. That's how the shift in generations was defined back then. Very drastic. Today, we mark generations far more specifically.