One of the ways I've stayed relevant in my fiction for teens and pre-teens was by being so much older than my two younger siblings. When I was in college and just getting the first inkling of an idea that I wanted to write novels, my younger brother and sister were in middle school. When I was out of college, they were in high school. Now, they are both in college and I listen to their problems when I see them, which is frequent, and remember all too well what I felt at that age. But now that they are a bit older, I needed to find new ways to connect with teens.
I love to give back. I try to help other writers out whenever I can and it's part of why I do Pitch Wars every year. In recent years, I've started to find opportunities to mentor a high school student writer or artist that wants to get into writing books or illustration. Some of these opportunities found me and other I applied to be a part of. I once had a high school student work with me on how to write a full novel and I found out about the opportunity through a local association emailer for writers in my state. They usually include job and non-profit opportunities for writers locally.
One way to stay relevant is absolutely by finding ways to volunteer locally. Get involved with teens through your local arts center, library, or just plain contact the high school and ask for opportunities to volunteer. Another method is doing what my screenwriting professor in college advised - eavesdrop and jot down conversations. Still, I think the best way to stay relevant to your readers is to actively engage with your age group you're writing towards whenever you can. The time I spent as a bookseller for teens and kids was invaluable because I actively got to ask what they were looking for and see what their parents wanted as well.
Either way, put yourself out there! You'll never know what sort of unexpected story might come from it.