Q: The rules of what constitutes cool in the jeans world is really too much for a man to keep track of. What cut? What wash? Dark? Plain? Bootcut? Stovepipe? Design on the pocket? Seven? Joes? Lucky? And then there is the question of age. Cool jeans should be different for the man of 38 than cool jeans for the man of 21. WTF?
A: Jeans are a lot like women. Some guys like blondes, some like brunettes, a few like redheads. Some like 'em skinny, some like a little meat on the bones. Some like big breasts, some like 'em a little smaller, some like 'em about the size of fully ripe Braeburn appl- ... anyhow, you get the idea. Point is, there are no hard-and-fast "cool" rules with regard to cut and wash (but always remember to play to your strengths).
About the second part of your question: Definitely nothing looks more ridiculous than a 40 year-old guy in a pair of Evisu jeans. Please consult the following age-jeans chart for guidance: