I sat next to three teen-age boys my second trip to see ROTK. I knew, of course, what was coming when the Rohirrim appeared on the hill. These boys were whispering on the edge of their seats and I didn't mind at all. During the charge, all three boys were still on the edge of their seats leaning forward and one was pumping his fist in the air:-)
Maybe young people, (at least some of them) are closer to these emotions than adults.
Continue to have fun. Your daughter has heart.
When I was my daughter's age, I was similarly hooked on Narnia. I can still quote you whole paragraphs from the books without looking. I didn't discover LOTR until I was a senior in high school.
The Rohirrim are tailor-made for all the horse-crazy kids, too. I prefer the lighter, finer Thoroughbreds, but my daughter likes the BIG horses, so she thinks the drafty-looking critters the Rohirrim ride are the best! Her favorite horse is a half-Percheron, half-God-knows-what named Shade. She looks like a baby perched way up on his big back.
The Rohirrim's horses actually make sense, the knights of old rode the Great Horse of Flanders and similar large weight-carrying breeds that were the ancestors of the draft horses. Crossing in the hot-blooded Arabs, Barbs and Turks gave us the Thoroughbred. Crossing the drafts back into the T'bred gives us the warmbloods.