For determining which generation a user belongs to based on their date of birth, the most relevant data validation would be:
Has the user inputted a date in the past?
This check is crucial because a date of birth must be a past date. Additionally, you would also want to validate that the format of the date is correct and that the month is a valid number (between 1 and 12) and that the day is valid for the given month. However, the most specific and important validation directly related to determining a valid date of birth is ensuring it is a date in the past.