These are somethings that we will always consider superior and best.
and by looking at these from an external point only can we see how 'old' we have become. We ourselves cannot realise.
eg. In our B-school time we enjoyed songs like Sharara and Shakira....but the next batch of our B school obviously doesnt have the same memories for these songs. And if we sit back and consider how old these songs actually are, we realise how old we are. But these songs will always bring a smile to our faces, no matter how late in years.
And so pleasant memories stay (and so do bitter ones) and act as a time determining factor...and are also the things that make you realise what you treasure.
Perhaps this will explain the nostalgic smile on your parents face when talking of something like the rage of XXX ... and our impatience and not understanding of why they had that half smile and lost expression.