I always go with Conan #1, but there is no generally agreed upon book, like Action #1 or Showcase #4 for the Golden and Silver.
75 is significant for Giant Size X-Men #1 because the X-Men was THE book of the Bronze Age. I don't know why you would think Iron Fist is significant?

Although it occurred in 1968, one of the clearest delineations between the Silver and Bronze Ages happens between House of Mystery 173 and 174.

Joe Orlando takes over as editor and the “Mystery” format starts.

Actually, the darker Batman was already happening under the Irv Novick / Frank Robbins teaming--the same behind the Grayson going to college issue you refer to (Batman #217 / "One Bullet Too Many!"), certainly leading to the Adams / O'Neil period usually cited as one of the foundational markers of the Bronze Age's start.

Further, the relaxation of the strict Comics Code mandates allowed a flood of more graphic / gothic horror stories to return to DC and Marvel, with DC really moving some of its titles in the mature direction of Warren (but not too far in said direction), all seen as part of the beginning of a new era--or age--in comics.

1970.

I don't think there is much to support X-Men as defining the Bronze Age and certainly not when Byrne comes on.  I don't even think you can make much of an argument for Byrne being the center of the Bronze Age, because it is far more than him.  X-Men turns into all reprints at the dawn of the 70s and continues that way until 1975.  The Bronze Age is clearly in existence well before Len Wein revives the series with Giant Size X-Men #1, with Dave Cockrum.  And, you ought to check out that era, because there is some great stuff there, not least of which is Cockrum's art.  His dynamic work has a lot to do with the revival sticking, especially when he introduces the Shi'ar.  Cockrum had a way with the space elements that I never felt Byrne could match.  Oh, sure, Byrne can do sci-fi; but, it's a different style.  Cockrum made it different and unique.  All you have to do is look at the transition between Cockrum and Byrne and see the Starjammer, as Cockrum designed and drew it and the rather generic thing that Byrne drew.  In Cockrum's hands, the Shi'ar are a truly alien race, with their own design aesthetics, rather like Babylon 5 did, with the various alien races in the show.

Iron Fist is a rather marginal character, even with Claremont and Byrne; so, again, I can't see how it is a defining moment.  You already have more prominent changes, in Superman and Batman, the Green Lantern/Green Arrow series with game changing art and story content that represents a shift into mature themes and social commentary.  Artists like Byrne and Perez take much of their lead from Adams' work on that series (and his Superman and Batman stuff), while a lot of writers take their lead from O'Neil.  You have shifts in other characters, too.  Jim Starlin comes onto Captain Marvel in 1972.  Steve Englehart and Frank Brunner are doing Dr Strange.  Roy Thomas and Neal Adams are doing the Kree-Skrull War.  There are just too many other mileposts before Byrne is even doing Wheelie and the Chopper Bunch, let alone Iron Fist or X-Men.

There is a clear change in style, in force, at the beginning of the 1970s.  1968 is probably a stronger argument, though I would say it is more the prologue, while 1970 marks when you get the main chapters.  Any "era" is always going to be fluid, because there is always a transition period that is hard to define, which is why such things usually are defined once the transition is over.  The Bronze Age marks Byrne and Perez' beginnings as major names; but, their real rise to fame comes with the dawn of the 80s.  Back at the time, there was a movement that defined the 80s as a different era from the 70s, using the rise of the independent publishers and the Direct Market as a demarcation.  There is a remarkable shift in the industry, at the time, as the rise of specialty shops, with a distribution system to feed them, gives rise to alternatives to DC and Marvel (and the other established publishers, most of whom were struggling).  You get creative ownership, which alters the relationships between creators and the Big Two, as they have a place to go where they keep ownership.  Royalties and creator rights come into play.  At the time, some started referring to the 80s as The Modern Age.  Problem is, there was another big shift in the mid-90s, with Image and the Distributor Wars and the Speculator Boom and Bust, not to mention Marvel's bankruptcy, at the end of the decade.  So, Modern Age came to define that reality and Bronze Age was now extended to include the 80s, before speculation went wild or before Image or when Marvel was riding high.  The 50s are a problematic decade, as it has elements of both the Golden Age and the Silver Age.  Earlier definitions kind of left the first half of the 50s as its own thing, rather than part of the Golden Age.

This kind of reminds me of the late 90s, when there was media talk and stereotypes of Generation X as "slackers."  There were books about "managing Generation X," in Business Publishing (meaning business books), which basically painted the entire generation as lazy underachievers who needed greater motivation than the Baby Boom Generation.  I used to bristle at this, since I was lumped in at the start of Generation X, yet I worked my butt off and my colleagues and classmates were of similar nature.  I also hated being lumped in with people who were in their early 20s and entering the workforce.  I am at the beginning of the usual start date for Gen X and my older brother is at the end of the Baby Boom, yet we both have similarities and differences with the two generational groups.  We experience Vietnam, culturally (since they weren't drafting soldiers THAT young), whereas those born in the 70s were not, really.  It was pretty much done before anyone born in the 70s was old enough to know anything beyond high chairs and cartoons.  I had much different life experiences than those young 20-somethings, because I was born in a transitional phase, where the Boom and Gen X overlap.  The ironic thing is that my older, Boomer brother fits more the Gen X stereotype (except musically) than I do.  He tended to be lazier and underachieve.  If you buy into birth order theories (and there is enough to argue against as for), then I developed a stronger work ethic and grades to stand out from him.  I don't buy that, as I never made a conscious decision.  I was never "Jim's little brother," so much as "Mr Nettleton's son," since my dad was a teacher in the school district (and my Mom had been, as well, before our births).  I think there is something to be said for defining transitional periods as their own entity.  That's why, personally, I always saw The Golden Age as the 30s and 40s and the 50s as a transitional decade.  The Silver Age, for me, really reflects the 60s, while the 70s marks another transition.  There is more in common with the work of the 80s and 90s, in my opinion, than the 70s and 80s and definitely the 70s and 90s.