Hi Robert! Each park has numerous rides and attractions that a 20 year old will enjoy! You can go through each park on the website,
www.disneyworld.com and you will find a list there. Or you can purchase a book at a bookstore such as "The Unofficial Guide to Walt Disney World" or the "Idiots Guide to Walt Disney World". Both are great resources that will describe the attractions as well as rate them for the different age groups.
My own children loved all of the roller coaster type rides such as Rock 'n Roller Coaster at Hollywood Studios, Splash Mountain and Thunder Mountain in Magic Kingdom, and Everest in Animal Kingdom. Other "thrill" type rides include Tower of Terror, Test Track, and Mission Space. But their enjoyment didn't end with speed rides....they totally enjoyed all of the 3-D movies such as It's a Bug's Life, Honey I Shrunk the Audience and Muppet Vision 3-D. Other ones that come to mind that are great as well are, Spaceship Earth, Soarin, Toy Story Mania, Buzz Lightyear, Pirates of the Caribbean, Jungle Cruise, American Adventure, Dinosaur, Lion King Show, Safari, etc etc etc. As you can see the list goes on and on and the majority of the attractions are geared for all ages.
All four parks are wonderful places to visit for a 20 year old. Have fun and enjoy your vacation!