Why does time fly by when your older?

I remember when I was 8 the years would go by slowly, very slowly, I'd be outside enjoying my childhood running to get ice cream with all my little cousins and even at 12 they'd drag of course I didn't mind it being summer for so long, or at least thats what It seems like. But now, being 18 and being a High School graduate the years FLEW BY like NOTHING! legit, I never thought High School would go by so goddamn fast. Freshmen year seemed like just yesterday, and can you believe it has already been TWO YEARS since Michael Jackson died:(<3 may his soul in peace, but why do years go by so much faster as you grow older:(