Every reader is different, and teenagers are good judges of their own reading level. That said, since graphic novels are by definition a visual format, they can contain disturbing or just plain adult images that are harder to skim over than similar context in a text-only book. Much like movies, many graphic novels have ratings that are intended to help readers gauge whether they are appropriate. As a general rule of thumb, if you see a book labeled M for Mature, 18+, or PARENTAL ADVISORY EXPLICIT CONTENT, it was not intended for a teen audience. Does that mean teens shouldn't read those books? Again, every teenager is different, and for the most part they can tell if a book isn't right for them. If you are a parent and are concerned that your teen may be reading content too advanced for them, try having a conversation about what they're reading -- or read the book together! However, keep in mind that reading is just about the safest way for anyone to explore a new and potentially scary or dangerous topic.
Every year, YALSA (the Young Adult Library Services Association) puts out both a longlist and a top ten list of graphic novels published in the previous year for their Great Graphic Novels for Teens award. If you're interested in seeing what books librarians serving teens recommend, either currently or in previous years, you can see their lists on YALSA's website.