Cell Validation Code (If Then Else) Not Working As Required

I have the following code to check the date of birth entered into cell C18 and to alert the inputter if the age is either under 16 or over 25.

VB:
Sub AgeValidation()
Dim age As Integer
Dim dob As Range
Dim AgeMsgAnswer16 As String
Dim AgeMsgAnswer25 As String

[Code] .....

It works to some extent in that the correct message box pops up if the age is under 16 and similarly if over 25. If answering Yes on either message box then the correct thing happens. Its what happens if the inputter selects No thats not right. If the age is under 16 and the user selects No in answer to "Is this correct?" then the code is clearing the cell contents and showing the calendar again but is also popping up the 'Over 25' validation message box which then won't go away until Yes is selected. Also there are then multiple copies of the calendar open.

What I need the code to do is look at the date selected from a popup calendar in c18 and decide if that age is within the 16-25 year old range. If it is outside that then the inputter needs to be alerted to it. I can't use the inbuilt data validation because there are some scenarios where it would be acceptable to have an age outside of that range but we want to cover inputting errors as well as double checking the age.

When a msgbox pops up to alert the inputter and they choose "Yes" to say the date of birth is correct then I want the focus to go to cell C20 ready to input the next piece of information.

When the inputter selects "No" on the message box, then I want the original date to be deleted and the calendar to reappear so they can select another date. So effectively resetting the field so they can start again choosing a date like when they first entered the cell.