Yeah, I spent most of my teen years writing stories with adult characters ranging from young adults to ancient beings, and when I read these stories now, they were clearly written by an angsty teen. Which is understandable since I was naive and didn't hang out with adults much.
I have to say though, emotional maturity and personality are two different things. I know a person who is younger than me (21) but is much more emotionally mature than me because she had to grow up fast in order to survive poverty and she had to deal with anger issues no kid should ever endure. But she is still discovering herself at this age because her personality hasn't fully developed yet. Her ideals and principles can still change. I can't pinpoint an exact age we all have our personalities established. Perhaps it's when we really know our limits, likes, dislikes, what we want in life and what about us is difficult to change. It's something to consider if you plan on writing adult characters.
There are so many things that can affect their change in behaviour and thinking, like hardships can harden someone and betrayal can turn someone into a cynic. It's things you discover by yourself or through other people's experiences (can be real people or fiction. The latter can be badly written, though). If you want to know how to write someone of an age group you've never experienced, it's always good to hang out with people from said age group, see how they behave, react to things and talk, and listen to their experiences because they're what made them into who they are now (eg. Relationships, jobs, their losses, their successes, families).
For example, I have a character who is a pregnant mum in their late 20's / early 30's but I have no idea what it's like to be one, so I had to refer to my sister-in-law. She's probably way more realistic than most depictions of pregnant women I see on TV, as many writers and producers tend to sugarcoat motherhood a bunch, lol. Which brings me to another point: don't always trust depictions of certain age groups and types of people you see in the media lol.