At least compared to your parents when they were your age. The reason is very simple, there is MORE to know! We know a lot more about our world than we did when you parents were what age you are now. There are also tools for learning that did not exist or was widely available to your parents when they were young.
Tools like the computer, and the INTERNET. You may have realized this yourself. That you know some things about how to use a computer, or the internet or even the TV remote that some adults have a hard time with.
Another evidence of your SMARTNESS is the fact that science and technology continues to advance every year. This would not be possible if we had to learn everything that was known about something and still have time improve it within our lifetimes, even though human life span has increased over the last several decades.
So there, I hope I have shown with these simple examples that you start out pretty smart. Now, the problem becomes staying smart.
Imagine if you were a very good and famous athlete. Would you stay famous and good if you stopped practicing and exercising, and started eating junk food and playing a lot of video games that did not require physical activity?
Staying smart requires exercising your brain, which includes reading, writing, doing homework. Yes, staying smart is a lot of WORK but it is definitely worth it.