I am 32 and I have met (and been) one of those teens that is more mature than some adults. And then you have teens that are teens and act as such or maybe they act younger. Everyone is different.
But I agree, it can be annoying when an adult says teens are less mature, when that particular teen is more mature then they are.
I think they assume because teens are younger, that they are less mature than adults. But that isn’t always a correct assumption.