We require all infants (children who have not reached their 2nd birthday) to be seated on the adults lap, with an infant seatbelt. A child can only sit in its own seat if it is able to sit completely upright, unassisted, with no slack in the seat belt. We do not allow infants to have their own seats (there is the odd occasion where an infant is still technically classified an infant, but bigger than some 4 year olds!- in which case I allow them to occupy a seat if one is free or has been purchased for the "infant").
Its because of this that I believe the definition of infant should be based not upon age but moreso upon size of the child. Admittedly this would be difficult to regulate, but given correct training Cabin Crew would be best able to make that decision - but then you have issues with parents wanting to/not wanting to purchase seats for their young ones... interesting topic, one that I have gone into some detail with our safety department about.
Perhaps could go on weight? Put the kids on the baggage weighing belt?
(just kidding)