Your line of thinking strikes me as a little funny because I had a job and started college at 17. I was deeply done with being a dependant and ecstatic about my freedom. I know a lot of young adults who lament adulthood but it's always sort of sarcastic. Woe, to understand consequences!
The worst thing about being an adult is society deeming you entirely responsible for yourself, your life, and your happiness. It is also absolutely the best thing. This doesn't become true at 18. It doesn't become true when you realize it. It's always been true. It's all on you. You and only you can make your life whatever you want to.
You're not crossing over some mystic boundary. Most other adults even four or five years older than you are going to still see you as a kid, especially if you are still financially dependent. Your late teens/early twenties are a great time to live your life to its fullest. If you think you've missed anything, take responsibility for living a full life and make it a mission.
Missions from my late teen/early twenties college years: 1. Choco Taco*: Get high after 8p and go on a mission to find choco tacos with no car. You may use public transportation, barter, and zealous recruitment of foot soldiers (other idiots) to locate the choco taco. *Can be any other rare/nostalgic item. 2. Spy Games: Explore your neighborhood/campus. See what you can get into/get access to without damaging anything. At one point my friends and I set up several inflatable mattresses, blankets, snack bar, and movie collection in a rarely used art room with a ceiling projector. 3. Tempting Psychosis: See how long you can stay awake. Document experience. Best done with friends while ignoring responsibilities. 4. Night Swimmer: Go to the beach/other outdoor space at night with flashlights. Explore. 5. Easter Bunny: Hide easter eggs everywhere in the wee hours. Wake up/stay up to watch people enjoy them. I was doing a lot of mushrooms at the time so I put candy and little slips of paper in them like, "You're loved" and "Share this with someone you love" and "Have a happy." I'm Jewish. 6. The Caterpillar: Bring a hookah, blanket, pillow to work. Set it up out back during your break. See if you can get your boss to join you. 7. And if I'm honest, lots of (safe) sex and (safe) drugs, but do your research first and do what's right for you.
The best part of your life isn't ending. It starts when you start it. <3