It appears that BA and VA will offer a free seat to the infant if said-infant turns 2 during the trip. Maybe OP's friend heard of such stories from other families traveling. Regardless, it's up to the traveler to understand the airline's rules...

https://www.aircanada.com/ca/en/aco/...hildren.html#/

To be fair, it's buried in the page as a caveat...

†† The child’s fare applies only to children under age 12. Youngsters age 12 and up will pay the adult fare. If your infant child’s second birthday occurs between the departure and return segments of an international trip, you have the option to pay:

• The full published child's fare for that flight (and be certain to have a confirmed seat both ways), or
• The infant fare on the departure, and the published child's fare on the return. Call Air Canada Reservations to book this option.

United is even more vague... https://www.united.com/en/us/fly/tra...-children.html