It's hard for anything dealing with age to be a true symmetrical bell curve - it usually winds up being cut short by old age & death, or foreshortened by, well, not existing. Notice that the ages start at 16. You'd have to be pretty precocious to give high quality answers at age 10 or 5.
Yeah, it is literally impossible for a real bell curve to exist, but if you want to be pedantic, that's also because people are clumpy. The bell curve might say there ought to be 0.00000067 excellent answerers at age 12, say, but reality insists on there being 0 or 1 excellent answerers aged 12.
Whenever we look at real data, we acknowledge that it's a discretized approximation to a bell curve and not a real (continuous) one; the point was that on top of the discrete approximation, we have the additional problem of anthropic biases - people not existing or dying at either end.