I think within ^this, the ideas of what constitutes a 'realistic' teenager and/or an adult should be carefully scrutinized, before making any further decisions.
Just from a societal standpoint, I think people tend to underestimate teenagers a lot. Not every teen is a horny idiot with a rebellious streak...that's a caricature that makes for a lot of fun drama on TV; it's not real life. =/
As someone who actually was a teenager who acted 'adult', I think it's insulting for writers and readers to insist that a person like me simply could not exist.
And many of my reasons for acting that way were extremely commonplace: for instance, being an introvert kept me out of a lot of petty drama and peer-pressure situations, because I simply wasn't interested. And being an eldest daughter from an immigrant family meant that I was expected to be responsible and make my parents 'sacrifices' pay off, while also taking care of my siblings and the household.
There was no life-altering trauma or fantastical setting that forced me to "grow up" before my time; it was just...normal life events combined with my personality, just like it would be for any other person. =/ There's nothing 'unrealistic' about it, and there's no reason another teenager, fictional or non-fictional, couldn't also be like that.
ALSO, and I think this is something really important: maturity and immaturity take different forms! It's not all the same stuff!!
To give an easy example: People generally tend to think of introverts as more mature because they are quieter, but that's actually not even remotely true. Some introverts are also shy: they are genuinely afraid of social interactions, and as they ACTUALLY mature and develop they can overcome those fears (I did).
Other introverts, especially those with autism, just don't get people. Part of the reason they avoid them may just be to avoid the uncomfortable situations that are bound to arise. But again, as they ACTUALLY mature, they can form new strategies for dealing with people and having smoother interactions. They can become brave enough to voice their needs to others and stand up for themselves, instead of running away or deciding that they just aren't worth people's time.
All that is to say, a character who seems 'mature' can still have room to grow; they can still be relatively immature compared to the people they will become as adults, or by the end of the story.
And If you're worried that your teenage characters are too 'adult like', maybe reconsider your notions of (a) what an adult actually is, and (b) whether their behavior just fits their character, before taking that seriously.
Like, are they actually acting like adults (who, believe it or not, actually behave in many different ways and also have various maturity levels), or do they just fit people's preconceived notions of what adults are like? Would making them act more like stereotypical teenagers add anything of value to the characters or the story? If you do want to show that they are still immature, is your current portrayal of the characters incompatible with immaturity...or have you just been made to think that it is...?