@Daniel8941 On Airbnb, they classify 0-2 years old as "infants". (That is of course ridiculous, as any of us who have experience with children know, a babe in arms, not yet mobile on their own, isn't anything like a 2 year old toddler who can climb up on things and get into all kinds of trouble if not constantly supervised).
Infants are not to be charged for, but that doesn't mean they don't count towards the maximum occupancy limit. This should be made clear on the host's listing- that all breathing bodies must be accounted for on the booking, whether an "infant" is charged for, or not.
Children over the age of 2 count as paying guests.
How guests are charged varies. Some hosts charge the same amount whether someone books for say, 2, in a listing with a max guest count of 4. Other hosts may have a base price for 2, with an extra guest charge for any over that amount.