This is an interesting post but the topic has drifted off the main question and into questions revolving around what you should do with your life. I won't presume to tell you what to do with your life because I don't know you... but I do have some tips related to keeping sharp.

1) Find your "sweet spot" of how many hours of sleep you need and make sure you get it. My sweet spot is right around 7hrs. If I go long or short of this for long periods I lose my edge. Apparently the number of hours required will fall as you age and if you have kids, well unfortunately you will be pulling short hours for a while.

2) Start each day with a regular routine. I use the Franklin Planner but the idea is sit down with cup of coffee without interruptions and decide what you want to get done today -- its called dayplanning in the Franklin system. Should only take about 20 minutes. Anything that needs to be done tomorrow or later that week forget about it, let it go and fully concentrate on the tasks for TODAY. (Yes, you need some kind of system to track things not necessarily the Franklin system but I like it and I recommend it.)

3) As an adjunct to dayplanning I do the daily crossword in the paper to get my mind going in the morning. You'd be surprised not just how that gets the morning cob webs out but also its a form of measurement of how sharp my mind is. Often I can tell from that if I'm coming down with a cold or subconsciously distracted by something that I need to deal with -- either can undermine sharpness.

4) I love coffee but too much is bad (esp in the afternoon) so watch the consumption of drugs... caffeine, alcohol, even OTC drugs like tylenol, advil, etc. Alcohol dulls my edge for 2 days if I drink more than a glass of wine or a beer with a meal.

5) Write a daily journal or diary. Its good to get all the ideas and emotions floating around in your head down on paper in black and white. You might be surprised at things you discover about yourself and others that you were just peripherally aware of until you write. Look for and correct any contradictions in your ideas and values since implicit contradictions interfere with integration and smooth operation of your mind.

6) Get regular exercise but don't over do it. Consider going low-carb. The brain operates equally well on ketones or glucose but many (including my self) find that low carb diet gives me more focus. Read Gary Taubes book "Good Calories, Bad Calories" for the real deal on the science behind low carb and the lack of science behind the modern diet offered by current medicine/nutritionist.

7) Make a distinction between a job and a career. A job pays the bills but does not challenge you intellectually. I currently have a job in IT that I like but its not something that fully engages my mind. Ideally you want a career but not everyone can get that. A career should be an open-ended field (i.e. lifelong challenges) that is commensurate with your abilities. If you have a job like me then you need to have other outside activities that challenges your mind. If you don't do this your mind will atrophy.

8) Read books and articles outside you main areas of interest and pickup other hobbies that force you out of your comfort zone such as learning a musical instrument or dance, etc. I meet new people in my job and I always like to ask them what do they do for a living, most people are happy to talk about themselves and I am always amazed at the things I learn from others.

9) Sadly, I must warn you that once you hit your late 30's or early 40's you mind just won't be as sharp. So all you youngsters out there do your hard thinking when you are young.

10) Make time for family, girlfriend, wife, etc but set clear boundaries so they don't interrupt or interfere with your thinking time. This is hard to do ;-)

Edit: Added #10