Mysteries of the fifth grade

Another article from Scientific American(August 1997)tells us:‘One of the fundamental mysteries of the fifth grade has at last been explained:scientists now know why Wintergreen mints give off flashes of light when you crunch them. ’
I do not know what ‘the fifth grade’ means here.Why not,say,the fourth grade or the Sixth grade?Is there anything special about ‘the fifth grade’?

Perhaps it’s something to do with the science curriculum for fifth graders. If not, then perhaps ‘fifth grade’ has been chosen because the author thinks it reflects a typical age when someone starts to notice the phenomenon.