I turned thirty last year. This is my third attempt to write up how this feels, and how it affected my mood for the year. The first attempt was too shallow, the second turned out too personal, and in this third try, I want to make it as simple as possible. In both failed attempts, there were two specific things that turned out to be the most important, so I will just get them out, and be done with it.
When you turn thirty, you know for a fact that you are not young anymore. It is possible to be mentally and physically agile and nimble if you take good take care of yourself (especially in your youth), but the piece of youth that you lose is the one about being a possibility, still having fundamental choices to make in your life. I believe that this unfinishedness, the possession of future potential, is what we most value in youth, what constitutes its aesthetic appeal. With thirty, the time credit you didn't know you had is gone, and now you're spending from the capital.
That was the minus side. The plus side is the following. You just finished the first decade in which you were on your own, and left to your own devices to make your own choices, without the restrictions of adult supervision. This last decade is now history, and now that you have a history, you also get to see a character in it. When you look back, you can't help but discern a contour to the cloud of decisions, mistakes and shimmers of cleverness. What's more, this character is even likable; he did everything for a reason, and these reasons are all that you hold valuable and interesting. He picked on the same shiny things which you can't take your eyes off of. You have become a character you wouldn't have thought of as your perfect future self in childhood, but one which is real and multi-layered and, just like any real character, hard to understand and understandable at the same time. Now you know through what kind of a character you will be living for the rest of your life, and in which directions you can force it to change.
There is much more to be said, but as said, if I don't keep it simple and short this time, it will end up being another dead post with a lot of text that just scratches at something it can't name. Maybe this is the joke on thirty: you will scratch it till fourty, just to get a new itch with a new decade.