This is a little tough to answer -
You entered it correctly.
They need to know the age(s) of anyone under 18 to make sure they have adequate space in the Kids Clubs or Common Grounds, or whatever.
She would be the third person in the cabin and pay less than if she were the second person.
But you are correct, the rate that you have to pay for a 15 year old as the third person in a cabin is the same rate as a third adult would be charged.