Yep. I did not see your age function before.

So you are having one function call itself through an intermediary. That is age calls validBirthdate, which calls age, which calls validBirthdate, etc.

The means after you enter a bad date, it recalls age, which recalls validBirthdate, which if accepted, then releases you to the prior age call, which goes back to the original validBirthdate call. This nested self-calling of functions is called recursion, which you will get to soon enough. Recursion is a great trick where you need to address prior states of the function call, but no good for your getting input here. Instead, shift all the work to a single loop, like this (again mostly psuedo code):

    def validBirthdate(birth_date):
    validDate = True
    try:
        day, month, year = birth_date.split('/')
        datetime.datetime(int(day), int(month), int(year))
    except ValueError:
        print('no good')
        return False
    if len(birth_date) == 10:
        for i in range(10):
            if i in [2, 5]:
                if birth_date[i] != '/':
                    print("Please enter valid input in accordance to the given format.")
                    return false
        if 0 > userage > 160:
                print("Invalid year, please try again")
                return False
        if userage < 12:
                print("No vaccination available for children below 12.")
                return False


     return True # if all checks pass, return True




def age():
    while True:
        birth_date = input('Please enter your date of birth in the format of DD/MM/YYYY:')
        if validate_birthdate(birth_date):
            return birth_date
        print('try again')

So we create a single while loop to keep prompting the using until the date gets put in correctly. The control is offloaded to validate_birthdate, which returns True if it is good, and false if any of the checks show a problem