In the below, cell B4 would be where you enter the desired age. So, for you,
B4 would have 5.
Then, the formula would go next to the child's DOB.
Abslute reference means that no matter where you put the below formula in
the sheet, it will always look at cell B4 for your entered age.
The $ before the B means always look at column B. The $ before the 4 means
always look at row 4. Having a $ before both means always look at B4.
Without the first $, if you copied the formula across a bunch of columns,
the B would become a C, then a D, etc.
same for the 2nd for rows.
Hope this helps!