(0, 12, 13,18,19) represent boundary values for the age groups. Boundary value analysis is a technique to test the values at the boundaries of an input domain or equivalence partition. explains this as follows:
Boundary value analysis (BVA) is based on testing the boundary values of valid and invalid partitions. The behavior at the edge of each equivalence partition is more likely to be incorrect than the behavior within the partition, so boundaries are an area where testing is likely to yield defects.
(-1,0,11,12,13,14,18,19) (B) are not boundary values, but include some boundary values and some non-boundary values. (4,5,15,20) are not boundary values at all. (-1.0,12,13,18,19) (D) are not boundary values either, as -1.0 is not an integer value.