No one thinks kids are old enough to be depressed. Apparently, depression isn't depression until you've spent a few decades binge-drinking and dating the wrong people. So, instead of being taken seriously, the common reaction a depressed kid will get from the adults around them is one of two things. You're going to get laughed at ("What do you have to be depressed about? You're a kid! Enjoy it! Just wait 'til you have bills!") or you're going to be patronized ("It's just your hormones/this age in general. Chin up!").
Even worse, those two common reactions constitute incredibly mixed messages. "Wait until you have bills!" implies that you should savor being young, because you'll only get sadder from here. On the other hand, "It's just your hormones and being in high school!" makes it seem like you just have to wait it out until the party that is adulthood starts and things will magically turn around.
Anyway, teenagers excel at a lot of things, like Snapchat and making the opening night of any movie at any theater complete and total Hell on Earth.
"The movie's been over for an hour; go home, you maniacs!"
The thing they excel at most, however, is understanding what's outside the norm. I was incredibly self-conscious about how depressed I was and how seemingly not depressed everyone else was, a fear the adults I trusted basically confirmed for me.