So perhaps this explains my issue trying to book a United Airlines flight from IAH to Tampico, Mx for July!
2 adults, 1 infant on lap, and one child age 2-4 comes to $2290
If I merely change the one child ticket to age 12-14 or 15-17, the total is $1897
If I change the child ticket to age 5-11, it’s $2290
Seems there’s a nearly $400 surcharge in this case if a traveler is under 12 years old.