My best advice would be, "Don't take yourself so seriously!" Learn to laugh, especially at yourself. When you get right down to it, we are all fantastically ridiculous creatures. Once you can do that, it is much easier to laugh and find joy at even the simplest things. And then you can experience life with the wonder that a child does.
I also believe that society's definition of "success" ages us well before our time. We spend so much time running around trying to acquire that we lose all of our joy. Whether it's fame, glory or money, if you can only measure yourself by what you have, you have a sad existence and you probably feel older than the calendar indicates you should.
Keep a simple life and try new things. I just turned 50, and in the last 10 years I've learned to ride a motorcycle, learned to scuba dive, have learned 2 new languages (Spanish and Portuguese), have traveled to a half dozen South American countries, taken up horseback riding and writing for HubPages. Who has time to feel old?