If you're a boomer, a lot has changed since you were a teen. For example, technology has advanced drastically, which has led to new ways of bullying. (I'm Gen X, not a boomer, but even with my generation, bullying was confined to school, the neighborhood, and occasionally phone calls that my parents could listen in on since the corded phone was in the kitchen. Nowadays, kids bully each other in school and the neighborhood, but also online, through apps, and through text messages and cell phone calls that parents are completely unaware of.)
But some things *haven't* changed. The basic concept of transitioning from childhood to adulthood, with the associated puberty, hormonal and body changes, trying to figure out one's place in the world, trying to decide whether peers matter more than family or vice versa, and on and on... The *external* trappings of teen-hood have changed, but the *internal* feelings and thoughts are, in some ways, very much the same.
So as others have said, read modern YA books. If you have any teens in your life (relatives, neighbors), talk to them about what their lives are like. See what is being written about for today's teens and what today's teens are actually living. Watch TV shows aimed at teens as well.
But also... you could write a YA book set in *your* teen years. Show what life was like in the 1950s or 1960s or whatever era you were a teen during, but also show that teens then went through some of the same issues as teens now. Would it sell? I don't know. But when I was a teen, I sometimes preferred to read books set in the past that showed that people "back then" dealt with similar issues to what I was dealing with. For me, at least, for whatever reason, those books helped me feel less alone than books set in what was then the "present day."