This paragraph, from Ryanair's information on unaccompanied minors, suggests that 16 year olds are considered adults for booking purposes.
Ryanair will carry unaccompanied minors aged 14 or 15 years ONLY upon completion and signature of a form of indemnity at check-in of EACH departure airport, by a parent or guardian. Children under the age of 14 years must always be accompanied on the same reservation by a passenger over 16 years. PLEASE NOTE FOR TRAVEL FROM THE 1st OCTOBER 2009 ALL CHILDREN UNDER THE AGE OF 16 YEARS MUST TRAVEL ACCOMPANIED BY AN ADULT (OVER 16 YEARS) BOOKED ON THE SAME RESERVATION.
And from their information on the carriage of infants, it suggests that anyone 2 years or over is charged an adult fare!
If the infant reaches the age of 2 years prior to their return journey, they must pay the applicable adult fare, taxes, fees and charges for that part of the journey.