I can't find anything that defines infant for this procedure but my guess would be any child young enough that they would need to do additional work to immobilize them. EncoderPros lay description says
"A minimum of two films of an infant's forearm are taken with the x-ray beam aimed at the midforearm. The infant or child must first be immobilized to prevent movement during the film taking. Films may be taken in the AP position with the hand supinated, in the true lateral position and in oblique positioning. "
The reimbursement is a little higher than 73090 which would possibly account for the additional work required when dealing with small children.
Just a guess on my part and if you find there is a defined age please post it, I would like to have it for my records.
Thanks
Laura, CPC