I've been curious about this for a while, though I really haven't found any kind of an explanation as to whether the choice of 17 as the allowed age for R-rated movies was completely arbitrary, or if there was something to make that happen.
Of course, now that people can go online and watch these movies at home or get them on DVD for their home theater system, I don't know if these ratings still apply unless one were to go to an actual cinema.