My mom tells me, "When you were a kid, you were so laid back and happy that we were worried you might be retarded."
In my tween years, I grew fiercely competitive. I always wanted to win and lead things. Any type of game I played in, I wanted to win. Every conversation, I wanted to be the smartest and funniest. Every group effort, out in front.
Now, in adulthood, I try to stake out middle ground that goes something like "be intense about things that matter, super laid back about everything else." In other words, be more intense about fewer things.
By shrinking the "stuff I really care about" box to just a few areas — off the top of my head: my relationships (friends and family), my work, and my personal development / learning — it allows me to focus intensely on those things and let go and/or be non-competitive on everything else.
A casual game of ping pong? Enjoy it. Political debates? Don't let it get too intense. It's okay not to win. Better, even, to listen and ponder while sipping green tea and staring pensively up and to the right. Non-core professional endeavors? Just suck less than the next guy, or outsource it altogether.
One reason I am less competitive the older I get is I see more situations as non-zero sum. I am more attuned to shared interests. Others don't need to lose for me to win.
Bottom Line: Maybe one part of growing up — oh, to grow up! — is picking your battles, winning the ones that matter, and seeing the others as much as possible as non-zero sum endeavors