Thank you very much for asking this question.
First, if you have a family doctor, I suggest your teen talk to him/her. Most doctors know something about depression. I started my road to wellness by talking to my family doctor.
I did not find any online support groups specifically for teens, but I found this for you, the teen’s parents. It’s from a reputable source.
I googled “Online help for depression,” and I got a very long list of sites that offer chat. Some are advertisements, and I did not look at those.
Mental Health America is the nation’s oldest mental health organization, and they have a chat room. I haven’t participated in this chat room, but MHA is the best. Click here
One of the best ways to learn to cope with depression is to talk to other people who have lived through depression. We call ourselves peers. Click here for a chat room manned by trained peers.
This one actually says it’s for teens and adults. That’s a good sign. I cannot vouch for its quality. Click here
Finally, here’s one from the National Suicide Prevention Lifeline. They are, of course, a superb organization. Click here
I have one very important suggestion. Talk to your teen openly and without judgement. Your teen can find millions of places online to chat about depression, but they will probably be of very low value. If any value at all. Ask your teen to tell you what sites they are going to in order to chat about depression. NOTE: never ask your teen to reveal the things they are chatting about. In fact, I recommend you tell your teen that their chats are private. Only ask them to tell you the sites they are going to.
I wish you and your teen the best of luck.