Something we definitely lose when we age is lack of judgement, and with it the ability to play and experiment. I miss that naïveté and over confidence from my twenties knowing I was allowed to screw up.
Is it really those things, or is it that as we get older we have something to lose? I could go live like I'm 20 again...reduce my spending to nothing, work all day on whatever I believe in...but that would cost me my family's comfort, and I rather like them.