DC Presents #26 (the insert, that is, not the main story) is probably the first real glimmer of the Copper Age.
Fantasy Quarterly #1, a 1978 book, is solidly Bronze.
But there are lots of "Bronze-ish" books being published, well into the 80's...Miller's Daredevil, for example, is cited as being "Copper" by many...but it's not. It's more the culmination of the Bronze age shift towards realism.
Ronin, though? Absolutely Copper.
Likewise, Dark Phoenix is the pinnacle of the Bronze Age form of storytelling. while Days of Future Past is most certainly Copper...in the same title, by the same creators.
We've had these discussions many, many times on the boards, and will probably have them forever.
There's no clear delineation for any "age", except for Action Comics #1. Everything else is qualified.