Toys/pillows/blankets in the crib are a suffocation hazard, especially while the child is too young to be able to move deliberately (not having developed enough motor control yet), and that's the argument against having them.
So you could say that once the baby has full motor control, it would be safe to add toys.
But (at least in theory) toys remain a hazard, so make a deliberate decision either for or against.
As Christine points out, older children can deliberately use toys to climb out of the crib - and fall down on the floor or onto other furniture. At that age, the toys aren't a suffocation hazard anymore because the child will move away from the object, just as adults would.
We have a similar question here already; you might find it worth reading.
Incidentally, SIDS has nothing to do with toys in the crib. It's called SUDDEN infant death syndrome because it has no apparent cause (or, the cause hasn't been scientifically discovered yet).