Note: I am not single, but I was, and intended to stay that way. I could happily be single again.
I think the key to being a happy single at any age is to be happy with yourself. Making a life for yourself that suits you, with or without a significant other is extremely important. The very first step is to know and accept who you are, all by yourself. When you don’t rely on outside influences (people or events) to bring you happiness, you are in the driver’s seat. You find your happiness through trial and error (and therapy, if necessary!), making a full life on the way. Having a romantic relationship should be the icing on the cake, not the cake itself, you know?