I was 6 when I began training. This was a good age to start in my opinion as it is said, children learn at a faster rate. However, discipline can be a problem.
I was too young to appreciate the "art" side of shotokan, as I was 6 when I starting training. My friend started so I went along and have trained non-stop for 13 years. Learning to protect myself was the option I chose on the vote as I think this was my earliest motivation