Since I was 10 I have had lucid dreams at the rate of a couple every few months - I am now starting to try to have some - I am almost 13 now. I have had maybe 20 but there are three that stood out so well that I still remember them.
The first I actually had when I was in third grade before I was 10. I dreampt that I was watching a movie about orange butterflies with blue shoes (don't ask) and I was absorbed into the movie. There was this earthquake and there were these bugs going around biting everyone and turning them into orange butterflies with blue shoes (something about changing their DNA). Well, they bit me and turned me into a butterfly. I knew I was dreaming and could reason it out, but it never occured to me to do something other than play out the dream life (which I didn't think of as a dream life at the time, for some reason).
The second I had a little less than a year ago. I was at my school waiting for my mom to pick me up, but the school was one huge construction site (it's not) and there was a huge cliff (there isn't). I had to go around somewhere to get to my mom at the bottom of the cliff and thats when it occured to me that I might be dreaming. I then told myself that if the guy around the next corner (somehow I knew there was a guy there, hmmm...) said "Don't go that way, its dangerous." then I could be sure I was dreaming. Sure enough, thats what he said. I decided to have some fun so I started flying around. After a while I woke up.
The third was the most interesting, I think. I dreamed I was at a birthday party. The people there were the same boys and girls as I had had at a birthday party when I was little except they were as they are now. Somewhere I realized it was a dream and so I got this idea to imagine I was a girl and then I started to change into a girl. I started to get into feeling physical changes or something. I swear I woke up, turned over, and fell back asleep several times and started dreaming again. It was really weird.