I agree w/ Rheinhard...wenn can mean "if" or "whenever" or "when", but not "then".
As I was learning German, I remember having whole sections on the differences between "als", "wann", "wenn" and "ob". They may not seem to a native German speaker to be too confusing, but because the English equivalents are, in some cases, similar, these can cause lots of headaches.
I think of it like Sebastian said, though..."than" is used for comparison (greater than, older than, less than, fewer than), and (instead of listing all the possibilities here) you can *almost* assume that then will be used.
To Ghol: ***Do not believe what you learned at school, when you were told it is pronounced 'thän'.***...I have to disagree. While most AE-speakers I know DO pronounce 'than' so that it rhymes with the English 'when', it should be more like thän...I try to use this to differentiate when speaking.