"The writing's on the wall" is more than just a Destiny's Child album from 1999.
But you probably knew that already. In fact, "the writing's on the wall" is an idiom that basically means, "doom and gloom are upon us, and there's nothing you can do about it." The phrase dates all the way back to the Hebrew Bible and the book of Daniel, in which supernatural writing foretells the fall of the Kingdom of Babylon.
Stevie Wonder, although he is quite a religious Christian, seems to be discouraging the paranoia implicit in "the writing's on the wall"-type thinking. He may not be quite right to call it "superstitious"—we think it's more "pessimistic" to use the phrase, since people aren't usually actually saying it because they see writing on a wall. But songwriters get a little artistic license. Plus, he's Stevie Wonder. That's probably enough said.
This is a baby with some seriously bad luck.
Thirteen has long been considered a mystically unlucky number, although the origins of the superstition are somewhat unclear. It might have something to do with Jesus having 12 disciples; it's likely, though, that the superstition dates back even earlier to ancient pagan religions with twelve Gods, like the Norse mythologies.
Breaking a mirror is also an ancient fear that has made its way into contemporary superstition: the Romans supposedly believed mirrors represented the soul, so it was bad luck to break them. Put all this ancient lore together, and it should go without saying that if you break a mirror, and you are a thirteen-month-old baby, you've got bad luck ahead.
How much bad luck? Seven years, obviously.
There are many mystical Christian beliefs about the significance of the number seven, deriving from its important role in the Bible. In Genesis, God creates the universe in six days and then rests on the seventh; in Revelations, the number seven appears in many places related to God's appearance on earth. Seven also has significant symbolic value in some Hindu, Jewish, and Muslim beliefs. No one is sure quite where the seven years of bad luck superstition got started, but if you believe in bad luck curses, seven is as logical a number as any.
If superstition ain't the way, then what is the way?
Although the song does not exactly make this clear, Wonder believed in following a Christian god instead. In fact, judging from other parts of his musical repertoire, it seems he probably though that those in doubt should go ahead and have a talk with God.