It’s called growth, and you’re transitioning. It doesn’t happen overnight, it takes months, if not years of incremental exposure to the outside world, experience, and emotions etc., to ‘adapt to the conventional expectations that comes with tranches of age.
Further, nobody really gives a flying Flamingo about your age and maturity. You’ll possibly get the occasional comment about how old, young, or mature you look for your age, but that’s all part of the process of growth.
I used to be irrationally scared of turning 25 because I had always been scared of the increasing rate of responsibility that progresses with age. I was scared that my perfect body was fading and that I was slowly losing the position that older adults envied because it meant that I wasn’t ‘old’ and could do things uninhibited without worrying about responsibilities, but life hits you quick and you’d have no time to romanticise about your fancy-free childhood or past life. In fact, you’d increasingly grow to love yourself and appreciate growth because it is accompanied with privileges, opportunities, and struggles that help you further GROW in the broader sense.
Just don’t let go of yourself in the process!