2. Don't ask the user to enter a string, use an int (1-12). Less to type, easier to compare. If the user types even one letter incorrectly for the month, it won't match.
3. To expand on salem c's recommendation, <ctime> includes a tm structure, holding a calendar date and time broken down into its components. This makes calculating a time duration like a user's current age MUCH easier. https://en.cppreference.com/w/c/chrono/tm
4. Calculating the user's current age: subtract birth_year from current_year, subtracting one year from that total if the current_month is less than the birth_month. int age = current_year - birth_year - ((current_month < birth_month) ? 1 : 0);