Just because a book is deemed "Young Adult" doesn't mean the genre shouldn't be enjoyed by adults decidedly older than the "typical" YA audience of 12 - 18 years-olds. Besides, anyone over 25 has a good sense of nostalgia and a lot of the recurrent themes in YA will take you right back to high school, sans zits and exam cramming.
Let yourself think outside your demographic (who are you to be pigeonholed, anyway?) and enjoy one - or more, or all - of these incredible young adult fiction books. Because you're an adult, and no one is going to tell you what to do.