First of all, Congratulations! I think moving into analyzing songs is like trying to analyze poetry. Sometimes there is a definitive message, and sometimes there is just the attempt of a rhyme or certain syllables, alliteration to fit with the rest of the message...you just don't know.
I loved this song in Napolean Dynamite so I thought I'd take a stab at it for you, and everything here is a metaphor to life. Not a literal dance, but the journey of life, right? and so is the race...He could have said if we win the race, ...but when shows a little more belief that we'll make it to the end, that it is possible, if he says "when." Teenagers do tend to think they will live forever, its a part of human growth and development that learned, anyway.
I understand it to be "Can you imagine what it will be like when we are old."
Also, this is a German band from the 80's- there might be something more to discuss, but I'm leaving it at that for now...I'm looking forward to reading other people's opinions here. Great topic!