Infant is a loosely defined word in the English language however Airbnb define it as less than 2 years old.
Airbnb's policy with regard to infants only allows 2 choices.
1. Not suitable for children under 2 or 2. Suitable but at no charge
As once in the distant past I was a young parent I sympathise with & wish to cater for parents and their infants but the reality is they are extra work.
A more sensible 2nd option would be
2. Suitable for infants under 2 on condition they use their own travel cot and linen.
This would not only encourage hosts to provide for infants but also...
a) Avoid liability issues possible with a host providing cots.
b) minimise the extra work would be restricted to waste disposal and guest access to laundry facilities.
c) It would not matter if guests were loose with the truth in declaring the age of their "infants" as the criteria would be based on whether they used the hosts' beds and linen or supplied their own.