Did you make your best decisions as a teenager? By the time you were 18? Few people would agree that they did. The human brain matures by around age 25, though we think of adulthood as beginning at 18 due to milestones at that age such as graduating from high school and being able to vote.
Food for thought: Help the kids in your life understand that they will be able to make increasingly important decisions as they get older and that you and other adults will be there to guide them, but that they will not "be an adult" until age 25. This helps set important expectations for kids. I have done this with my son, who is now 7 and very excited to get older, and it has led to some good conversations about the decisions he gets to make, how he is making many more decisions now than he did even a couple of years ago, and when he will get to make certain decisions he is particularly interested in (such as setting his own bedtime!).
Here is more reading on the topic from the National Institute on Mental Health:
JoAnne McFarland O'Rourke