My philosophy is simple...
Growing older is a must but growing up is something entirely different. Why be old and grey before your time? Stay young by livivng for today and not yesterday or when you were... and life becomes a continual voyage of discovery and wonder rather than a boring dull experience.
Also you know you are old when you can still remember playing the classic games on Abandonia on the machines they were originally designed on. (when I was your age alley cat was cool...)