I was twelve. I thought the winning answer was Professor Plum in the Conservatory with the lead pipe. Nobody could disprove it, so I got to look in the envelope. Unfortunately, two out of three counted as a loss.
Now I'm thirty-four. I think Matthew kept an intermittent journal during Jesus' ministry, which supplied much of Mark's material when he wrote in the early 50's AD. Then, in the late 50's, Luke used Mark's Gospel and Matthew's notes, plus first hand interviews, while living in Caesarea. By the early 60's, conflict between diverging streams of Jewish-christianity in Judea inspired Matthew to convert his own notes into a full composition, also borrowing material from Mark and Luke.
I'm pretty sure nobody can disprove that. Naturally, it's also unprovable. Now if only I could find that dang envelope! ;)