I'm 21 and took my first (mostly) solo trip in May. I say mostly because a friend met up with me a few times during my trip, but the majority of my trip was spent solo.
Was it different? Yes. Was it weird? At times. But it was a LOT less awkward than I thought it would be. To be honest, the only times I really missed having someone with me was when riding Buzz (competing against my old scores was fun, but competing against someone else is more fun), waiting for Festival of the Lion King to start, on the busses to/from the parks, and swimming at the hotel. I can also see how eating would be awkward for some people, but I tend to not eat much and ate at non-peak times, so I was ok with the whole eating thing.
It was really nice to be able to do exactly what I wanted to do, when I wanted to. I could take all the time I wanted to take pictures, without holding anyone up. I could get a pineapple float for lunch, and not have to worry about whether or not someone else wanted something different. I could ride Splash Mountain as many times as I wanted. When dinner made me sick one night (my fault, not Disney's), I didn't have to worry about traveling companions missing out on fun; I dealt with the issue and returned to playing when I was ready. Oh, I also went to 4 parks in one day. Probably wouldn't have been able to do that with most of the people I've been to Disney with.
I guess what I'm trying to say is there's total freedom when you're solo. And Disney IS fun as a solo. Plus Test Track and Everest have single rider lines. That in itself is a HUGE perk. I was lucky. I had total freedom for the majority of my trip, but also had times where I wasn't by myself, which was also wicked fun. And don't forget, when you're solo you're not arguing with the rest of your family or friends