I read Watership Down for the first time in my 20s, too. I had heard of it as a child, but for some reason a bunny book didn’t appeal to me. I can only conclude that I was a wrong mixed-up little girl; it’s now one of my favorite books, and every time I read it I find something new.
I’ve never read any Beatrix Potter books; the cutesy illustrations turned me off. They’re creepy, you say? :makes a scribbled note on library list:
A friend of mine had never read Green Eggs and Ham (or any Dr. Seuss, actually) until I made him do it last week.
He’s 21.
That’s mine. I read it for the first time about 6 months ago. Just after the new movie (which I still haven’t seen) came out.
I, too, was in my twenties before I read “The Secret Garden.”
I began reading the earlier, more obscure Seuss books as a kindergarten teacher: 500 Hats of Bartholemew Cullins, The Seven Lady Godivas, McElliots Pool; also, his posthumous books, like “Hooray For Diffendooer Day!”
I bought the somewhat discredited, “The Story of Little Black Sambo” two years.
I read “Goodnight, Moon,” for the first time ever two weeks ago, to my niece.
I was not big into fantasy as a child, so The Hobbit and Lord of the Rings are both fairly recent reads, both of which I enjoyed (although LotR took a long while to get through).
I just finished the entire “Chronicles of Narnia” series, another one I didn’t read as a child; I mostly enjoyed the stories, but was surprised by the violence and killing - I don’t think I would have liked it as a youth. My favorite story was probably “A boy and his horse.”
Look for The Tale of Pigling Bland,* or Little Pig Robinson.* Beatrix Potter likes to make bad things happen to pigs. There’s also The Pie and the Patty Pan, on why cats and dogs should never have tea together, and Roly-Poly Pudding, to which a summary really can’t do justice. The problem is, she takes anthropomorphization to it’s logical conlusion.
You mean “The Horse and His Boy”, Humo-centrist! 
I read The Chronicles of Narnia earlier this year, and thoroughly enjoyed it. I found myself wondering how I missed these books when I was younger.
All of Shel Silverstein’s author pics are damn creepy.
Boy, my coding sucks. Anyway. I’m not going to remember the title, but there was also one about the pitfalls of offering credit to rodents. Yes, I mean allowing them to purchase goods without paying cash upfront.
A Wrinkle In Time, and Heidi. Heidi makes me hungry for cheese.
The Tale of Ginger and Pickles, I think.
I read The Lion, The Witch, and the Wardrobe in school as a kid, but wasn’t even slightly motivated to read the others (which was remarkable, because I was the sort of child who usually snapped up everything in sight). I tried to get into them when I was in college, but only made it through two or three volumes before I decided that my childhood instincts were right.
I read Eloise for the first time when I got a job in a children’s bookstore at twenty-three, which may actually be the right age to appreciate it. (That was also the summer I discovered the Harry Potter books. That was a good job.)
I read *The Yearling * as an adult. Actually I’m not sure that qualifies as a kid’s book. Old Yeller is a laugh riot compared to it.
Oh yeah. I read The Yearling not too long ago, too. Good call.
I read (for the first time) Charlie and the Chocolate Factory to my son after he saw the movie. We’re about halfway through Charlie and the Great Glass Elevator.
All I have to say is the Mr. Wonka is a psycopath and Charlie’s grandparents, including Grandpa Joe, are louts and worthless individuals. Get your lazy asses out of bed, dammit.
I really enjoyed them when I read them this summer, oddly. They just made me happy. I actually ended up doing an essay on the first book.
I read the Narnia books when I was in grad school, so I totally got all the Christian symbolism.