MAP = Middle Aged Photographer. The time at which a crisis is likely to emerge. If we're lucky the crises will involve nothing worse than decisions related to exposure, processing or whether to quit our day jobs.
13 = The age intervals at which crises are likely to occur, i.e.:
"I'm 13, yippee! What! Now I'm a man? Yikes!"
"I'm 26, I'm more than a Quarter Century Old!"
And so on.