Although I don’t believe such a thing exists on a scientific level, here are some symptoms I can imagine. 
Consider yourself in a mid-life crisis if -
- You are getting nostalgic about the past lately
- You think cutting your hair short makes you look younger
- You think tight T-shirts suit you better
- You get unusually happy when you buy from Forever-21 or American Eagle or Hollister or Abercombie.
- You start believing in stories of young girls dating married old men
- You say you don’t remember your age on your birthday
- You start becoming ambiguous about your age in general – you prefer to use floor() instead of round() on your age. You fight with all your liar’s might to stay below 40.
- …..
My friend Troy has contributed more to this list… his list is in his comment. Thanks Troy for your contribution!
That’s so funny.. my wife keeps telling me I must be in “one-third-life-crisis” rather than mid-life crisis… several items on your list apply to me.
You could add to your list..
- You just bought a motorcycle
- You start working out, because you think it might be your last chance to actually be fit
- You are more motivated to finish your projects before your mental faculties deteriorate or you get too lazy
- You find sites like Facebook annoying because you don’t want to keep up with the latest tech fads all the time
- You realize you’re closer to 40 than to 30 and are momentarily disturbed by that thought
On the bright side… I have a lot more energy lately.. and value my remaining time more highly.