Second, everyone grows up, despite the Peter Pan myth and the notion of living forever. No one wants to grow old or stop loving the things they love now; think of all the people who have mid-life crises, for example, and buy Harleys. In Florida we have college Quidditch teams and recommendations for the next Game of Thrones book.
Then we have toddlers wearing high heels and posing as Julia Roberts; parents have a reason to be concerned. As noted, we can't stop getting older, but we can certainly not try to speed up the process. Maturity and actual growing will only happen if you stop and let it happen.
Some days I get frustrated because a part of me likes to slow down in the morning and read the latest library book, even when I want to beat the morning traffic and be efficient. That same part of me is the part of me that remains young, the part that allows me to make fun of myself. I can put that laziness (or appreciation for the fine arts, if we put positive spin on this habit) into my stories and create vivid, realistic characters with similar flaws. I can also use it to bond with other people who may have similar habits.
These same people share a mutual love for the same movies and music, even if these movies aren't aimed at our demographic. For example, I have memorized every song in the Little Mermaid, and I'm not ashamed of it. At the same time, I've grown out of shows like Teamo Supremo and Barney; some nostalgia doesn't last. These loves and dislikes balance out so that I will watch Wishbone and Aladdin on Youtube.
Another part of me likes career planning and being on time for school, if not for class. The two are quite different: being on time to school means finding a parking space in the closest lot, while being on time for class means beating the large grandfather clock. I admit that this is the adult side of me, because now I no longer care if I'm on time for my first class and have to relearn it with the fervency of a middle-school student. Again, I can put that in my books because I know kids who used to show up to college classes in pajamas.
Maturity happens when you have to be proactive. When I started to study marketing and aspects of business, I learned that to get any job you have to check application deadlines and meet them as early as possible. I was earlier proactive as a writer after getting disqualified from a writing contest due to a format error. (Yeah, that was not fun.) Now I revise cover letters and resumes ahead of time, much like how I revise query letters and short stories. On some level it means acknowledging that my dream of being a full-time writer may not happen for a long time, but it also means that I'm ready to rough out these years of rejection until one acceptance leads to another. I'm ready to take risks , but I'm also ready for if the risks bail on me.
I'm a young adult. People tell me I'm still young while I worry about my future, and the "adult" part of me reminds me why I should worry. Like my shoulder angel and devil, these parts of me can come into conflict. Part of the fun, however, is the bickering between them; although I may be getting older, parts of my personality will not change no matter what cynical influences appear in my life.
It's more than okay; it makes living an adventure with good and evil on your side. You get to be arbitrary about it until the odd day that maturity wins. And like all things that win, maturity will lose on the even day, but it won't give up.
Neither will you.