Indeed?!
I had always had the impression that they were a generation older (thus way predated) Constitutions!
Although, I did once in a Star Trek comic book, I forget if it was Marvel or DC's, but Capt. Kirk did transfer onto the Big E from the U.S.S. Republic, a Baton Rouge looking kind of ship, when Captain Pike was stepping down as the Enterprise's captain. But I wonder if a comic book can be considered canon!