Each check-in counter has a flight designation. When checking in for the flight, passenger is given flight coupon – boarding pass.
Carriers apply check-in per flight (check-in of certain flight at designated counter).
Check-in counters for international flights open 90 minutes before scheduled departure time.
Check-in counters for international flights usually close 30 to 40 minutes before scheduled departure time.
At your airline you can be informed about the exact time of closing of check-in counters before departure.
When boarding an aircraft each passenger should have a boarding pass with him/her, irrelevant of age.