As far back as I remember I wanted to serve a mission.
One of my earliest memories is from when we lived in Valparaiso Florida. My dad was a Captain in the Air Force and was stationed at Eglin Air Force base. He served as the Branch President there as I recall. The missionaries would sometimes come over to our house on their Preparation Day (I don't know if that's what they called it back then--this would have been around 1960.) I don't remember too much about them except that we had some inner-tubes we used when we went swimming, and I remember the Elders could throw the inner-tubes really, really high up into the air and when they came down they would bounce them right off of their heads! I thought that was the coolest thing ever! I always remembered that, and as I grew up I wanted to go on a mission so I could be like those Elders and bounce inner-tubes off of my head!
Of course that is not what serving a mission is about, but it left such an impression on me that I can't recall a time when I didn't want to serve a mission. This was never a question for me growing up--I thought missionaries were cool and I wanted to be one of them.