The value you are trying find the youngest age is considered string type of variable. The minimum method requires numeric values. The "/" in the value makes it a string. You can either alphabetically sort the data or convert the dates to sequential numbers using the JavaScript Date object. Have you tried different entry values to see how the age field fills in?
Entering a value of "1/2000" will display as "01/2000" but have a value of "1/2000". This missing leading zero could cause a sort problem.
See YoungestDateValues to see the results of 4 different ways to perform the task. Only 3 will provide the correct result.