Here is the text from a typical EY F fare rules
Code:
Category 19: Discounts
ACCOMPANIED CHILD 2-11 - NO DISCOUNT
OR - INFANT UNDER 2 WITHOUT A SEAT - CHARGE 20 PERCENT OF
THE FARE.
TICKETING CODE - BASE FARE CODE PLUS IN AND
PERCENT OF FARE PAID.
NOTE -
INFANTS WHO REACH SECOND BIRTHDAY BEFORE RETURNING
TO US MUST PURCHASE A SEAT FOR THE INBOUND FLIGHT-
CHARGE HALF THE INFANT FARE OUTBOUND IF TRAVELLING
OUTBOUND WITHOUT A SEAT AND HALF THE CHILD FARE
INBOUND.
OR - INFANT UNDER 2 WITH A SEAT - NO DISCOUNT
I booked 2 adults and 3 under 11 year olds in F last year and kept fiddling to try to get (what is fairly routine) discount (usually 20-30% depending on the carrier) for them but never could. You can see in the fare rules that they don't offer that discount. So 20% charge for lap infant and no discount for under 11 year olds...