I took my 18 y/o brother to Germany when he graduated high school. Touring these castles is something we both wanted to do. It was nice and easy planning, especially for a first time (and probably last, but still) trip to Germany. Perfect because we didn't have a rental car. And if we had a rental car, we would've missed out on a lot of the history of not only the castles, but also the surrounding area. It is a beautiful, scenic drive. Frankly, well worth it. We were able to squeeze in a walk up to Schloss Hohenschawngau as well. Trying to see everything makes it a busy day and we didn't get to spend nearly enough time at Schloss Linderhof. We both thought that if you had the time, you could rent a car and do a several day driving tour of the region, seeing the castles. Oberamagau was also a nice stop and a great place to pick up gifts for people back home.
The take away is, even the 18 y/o boy had a good time.