I like to think of life as having three principal segments;
0-30 = young
30-60 = middle aged
60-90 = old
Above this, very old.
Haha, I'm pretty sure most 30 year olds would be pretty shocked if someone called them "middle aged"! I realise it's not a clearly defined term, but I think it tends to be associated with people around 45-55
I think you should act in accordance with your age, life circumstances and responsibilities. For example, a 19 year old with a baby shouldn't be out clubbing every night because it's irresponsible, however if you're 30 with no kids then that's up to you as you're not harming anyone.
It's hard to say really. It is quite the frame of mind!
I'm 26 and I don't feel very young because of what I am looking for in life right now, but I'm sure there are those 10 years older who have a much younger outlook.