I'm 40 and in the best shape of my life workout 6 days a week, have all my hair, etc. Do I wish I was 25 or so? Of course but I don't feel like it's downhill yet. Get back to me in 5 years though.
I still play competitive basketball against kids half my age so that helps me feel young. Just a little more sore the next day. Life is what you make of it, I know 30 year olds that look like and act like they are 50. Not for me...