Source- Elines 

Due to an increase in the number of on-board incidents involving animals boarded in the cabin, United Airlines announced the following
changes to the Emotional Support (ESAN) and Service Animal (SVAN) policies, effective for all reservations made on or after Monday, January 7, 2019. 

The decision not to accept for carriage puppies and kittens under the age of 4 months is due, in large part, to younger animals typically not having received the necessary vaccinations that help to ensure the safety of employees and customers. 

Note: Reservations made prior to January 3, 2019 will be honored under the previous policy with approved documentation.

Additional information can be found at United.com > Travel information > Traveling with animals >Service animals or by accessing your Policy & Procedures dated January 3, 2019.