Probably the biggest difference is in the brain. Just like everything else in your body continues to grow until your early twenties, your brain is also still developing. The last part to develop is your judgment center located in the front part of our brains. In the past 15 year olds weren't required to make alot of serious decisions so the brain and body developed all the other parts before completing that area of our brains. As a result when we are younger we often make decisions that we really regret later in our lives. The bad part is that it is nature's cruel little joke because at the time it really seemed like a good decision. I know I made a few that I really wish I could take back. Trust yourself enough to wait awhile on the really big decisions. You'll be twenty before you know it. Even though it seems like a long way away right now.
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A 15-year-old is typically in high school and still considered a minor, while a 20-year-old is often in college or the workforce and considered a legal adult. The 20-year-old may have more independence, responsibilities, and life experience compared to a 15-year-old.