У вашего броузера проблема в совместимости с HTML5
import datetime
Year = int(input(" Please enter the year you were born "))
Month = int(input(" Please enter the number of the month you were born. For example 3 = March "))
Day = int(input(" Please enter the day you were born "))
DOB = datetime.datetime(Year,Month,Day)
Age = (datetime.datetime.now() - DOB)
print("You are " + str(Age.days) + " days old")
convertdays = int(Age.days)
AgeYears = convertdays/365
print("Or " + str(AgeYears) + " years old to be less precise!")