Reading is something you do for pleasure, so read what your heart pleases! I love YA lit, but I am definitely no longer a young adult, at least as far as the genre is concerned. You do not need an excuse other than you want to to read a book.
After reading many distopian novels, I'm not as impressed with The Giver as I should be. I kept thinking of other novels, some of which were written after The Giver, so it makes it hard to enjoy the book. I will read the other books simply to find out why people are naming their kids Gabriel instead of Jonas (read the forward by Lowry).
My friend made me read this book because I was making fun of her for reading it, and I am so glad she did. As a middle school student who had read just about everything on my reading level I found interesting, this book reminded me why I like to read. I encourage every young reader to read this series because it teaches some valuable life lessons.