I think they include Ben Franklin's generation. I actually have the book at home (both "Fourth Turning" and "13th Generation"), but forgot to look at them last night.
Other popular terms are "Thirteeners" or "13th Generation", which come from a book by Neil Howe and William Strauss called "Generations", in which we are listed as the 13th generation of the USA since 1620.
From
http://users.metro2000.net/~stabbott/genxintro.htm
so I guess this start with the Pilgrims. Interesting.