When you're 10, five years is forever.
When you're 20, it's still a long time.
When you're 60, five years is like a long weekend.
Daniel Tammet, the famous mathematician wrote an interesting book about this very subject called 'Thinking in numbers".
It - in part - describes how summers feel like forever when you're young because, they almost are, relative to how old you are. A 3-month summer when you are 5, is 5% of your whole life, spent in that summer. When you're 50, it's just 0.5% of your life and thus it feels like it has gone that much quicker because your perception of time has change.
It's a good read.