If BO was only 4 years old I could understand the term "infant". But according to what was posted, in 1967 he would've been 5 or 6. As Candor7 explained it seems the proper term was "infant". It just sounds strange to me. But everything about BO is strange.
Many people still consider Obie to be acting like a baby even though he is (possibly) 49 years old...