A) They interviewed 2,400 persons who were 15, 16 or 17 years old;
B) They interviewed 2,400 persons during the past 15 or 17 years; or
C) Something else.
Because none of it makes sense in the context...
ETA: The context: a study which has been made annually since 1979 and this is from the report for year 2006. It's supposed to research people's interest in investing and borrowing in various ways.
I first thought that sentence meant A), but how could they interview 15-17-year-olds about investing and assume that's enough to map the whole country's investing behavior? So I hoped I had misunderstood the sentence (because it sounds a bit weird to my non-native speaker ear) and that someone could tell me if it meant something else.
ETA2: I probably should have thought of this before, but the comments on the sentence being awkward gave me the idea that it could be translated. It was. Apparently by someone half-asleep. The original version of the article was found after some searching, so I checked what the sentence in it was like. Loosely translating: "2,400 people between ages 15 and 74 were interviewed." So apparently it really was referring to their age... (But how do you accidentally write "17" instead of "74"...?)
To everybody: Thanks for your help! It's reassuring to know that it wasn't my fault that the sentence seemed so difficult to understand. Apparently my English isn't getting that rusty.