[Markus' summary of his question: "Well, I finally asked Mr. Martin to confirm the date of THK, and pointed out that Mormont's history lecture in ACoK was a bit confusing. I also asked him about Tyrion's age, but he didn't answer that."]
The key word in my "about a hundred years" is "about."
"The Hedge Knight" takes place around 208-209, as you surmise. I have the exact year in my notes, but I don't have them to hand at the moment.
Mormont's dialogue -- and the dialogue and thoughts of other characters, for that matter -- needs to be understood =as= dialogue. When we talk, we tend to be imprecise about such things, saying something happened "in the sixties" or "at the turn of the century," or that World War II was "fifty years ago." It's no different in the Seven Kingdoms.
And that goes for distances as well as dates. A phrase like "a thousand leagues" is not meant to be a precise measure of distance, only the equivilent of "a million miles away," ie, "a very long way."