C++ age calculation function

I have written code to get user input for their birth day, month, and year (all ints). All of that gets put in a struct called Date which I initialize in main with Date input for the birth date and Date output for the desired date. therefore to reference them you would use input.day, input.month and input.year and so on using output instead of input. What I need now is a function to take these structs and calculate the users age based on their input. You can just call the function CalculateAge.