Tbh it’s because of the expectations and the pressures that come with the concept of age as a whole. Don’t get me wrong, age is a real thing — your brain development is very real and is very different from ages like six compared to ages like seventeen, even thirteen and sixteen are very different development wise. But there’s the scientific concept of age and then the societal concept of age, and one of them is based purely on opinion. I think we can all figure out which one that is LMAO
I turned 18 earlier this year and pretty much had a whole crisis around feeling like I suddenly had to move out, get a job, take on responsibilities that I’ve never had to take on before (mainly due to mental health issues and whatnot) and it was impacting me to a really wild point. I also was struggling with feeling like I was too immature for the people I hang out with (who are only a year older than me) and that I was someone people were never going to take seriously because no matter how hard I tried to “act my age”, I never… felt my age. Which sort of begs the question of what it even means to feel your age.
Now, personally, I have a bunch of opinions on the concept of being told to act your age or being told you have to feel a certain way by a certain age, and I think it’s very… non-inclusive of many different groups and communities in society and actually operates with the intention to make people feel bad and ashamed, as well as an excuse for some people to get away with some things, but I won’t get too heavily into that 
Truthfully, the only important aspect of age is the science behind it and how you let that interact with you. Like, and this isn’t me accusing anyone of anything, I’m just using this as an example, people who feel younger than they are definitely shouldn’t try to hang out with people many, many years younger than them. Especially since, regardless of how alone we might feel, there’s always people out there, same age, or close enough to it, that feel the same way.
But yeah, the easiest way my friend explained it to me is age isn’t really a feeling. You can associate it with certain things and feel like you have to hit certain milestones by those points, but you’ll never actually feel a certain age. Just remember we’re always changing and always progressing, even when it doesn’t feel like we are. And when you’re stuck in a society that tells you certain ways you act don’t fit their idea of maturity, you’re bound to start ignoring the main ways you’ve progressed (mentally, emotionally) simply because people around you only acknowledge the ways they feel you haven’t.
I stopped associating my ‘immaturity’ with my childhood simply because you don’t naturally stumble upon emotional maturity, you don’t wake up one day and suddenly gain all the abilities society tells you that you need — you have to work towards it and start making conscious efforts to learn. Especially if you have a lot of trauma. And if your ‘immaturity’ comes from enjoying life and having fun, I think it’s more important to ask yourself why your joy matters that much to others when it’s not hurting anybody. Just be yourself, find ways to enjoy life that isn’t harming you and isn’t harming other people, and exist the way that best suits you and helps you find contentness in the world. We’re all just little creatures in a big universe trying to get through this unexpected little life, you know?