current_year = 2016
year_of_birth = int(raw_input('Enter Year Of Birth: '))
age = current_year - year_of_birth
mytext = 'You are %s years old.'
print(mytext % age)
if age < 18:
    print('YOU SHALL NOT PASS!')
else:
    print('Welcome To The Portal.')

from datetime import date
current_year = date.today().year

def ask_for_birth_year():
    while True:
        try:
            return int(raw_input('Enter Year Of Birth: '))
        except ValueError:
            print('This is not a number, try again.')

def ask_for_birth_year():
    while True:
        try:
            nb = int(raw_input('Enter Year Of Birth: '))
            if nb < 0:  # can be any condition you want, to say 'nb' is invalid
                print('Invalid year')
            else:  # if we arrive here, 'nb' is a positive number, we can stop asking
                break
        except ValueError:
            print('This is not a number, try again.')
    return nb

mytext = 'You are %d years old.'

if __name__ == '__main__':
    # do stuff

#at the beginning
LIMIT_AGE = 18

#your 'if' statement
if age < LIMIT_AGE:
    ...

from datetime import date


LIMIT_AGE = 18


def ask_for_birth_year():
    while True:
        try:
            nb = int(raw_input('Enter Year Of Birth: '))
            if nb < 0:
                print('Invalid year')
            else:
                break
        except ValueError:
            print('This is not a number, try again.')
    return nb


def print_message(age):
    mytext = 'You are %d years old.'
    print(mytext % age)
    if age < LIMIT_AGE:
        print('YOU SHALL NOT PASS!')
    else:
        print('Welcome To The Portal.')


if __name__ == '__main__':
    year_of_birth = ask_for_birth_year()
    current_year = date.today().year
    age = current_year - year_of_birth
    print_message(age)

from datetime import date
current_year = date.today().year
year_of_birth = int(raw_input('Enter Year Of Birth: '))
age = current_year - year_of_birth
print('You are %s years old.' % age)
if age < 18:
    print('YOU SHALL NOT PASS!')
else:
    print('Welcome To The Portal.')