At the airport, babies (i.e. up to two years old) are not passengers like the others. Special rate, luggage, priority boarding… The conditions vary greatly depending on the company. The Skyscanner flight comparator has made a point for us.
If some companies like Air France (for flights in mainland France) or Air Canada (flights in Canada or to the United States) offer the trip to babies, most charge for it. For many companies, the price is set at 10% of the adult ticket price, but this can go up to 20% (at Thai Airways or Emirates for example). Finally, for some companies like Transavia, easyJet or Vueling, the price is fixed (€20 for Transavia, €26 for easyJet and from €5 to €40 at Vueling + airport taxes).
If all the companies allow you to take luggage with the baby ticket, you are not always entitled to the same things:the minimum is to be able to carry a piece of luggage of 10 kg. We sometimes have the right to more, such as a stroller and/or a car seat. The best thing is to check with the company you are traveling with, to be sure not to pay any extra.
In order to avoid an endless wait in the boarding lounge, some companies allow passengers traveling with a baby to board first. This is the case of Air France, Brussels Airlines, Corsair, easyJet, Emirates, Lufthansa, Qatar Airways, Royal Air Maroc, TAP Portugal, Turkish Airlines and Vueling.
The complete list of possibilities by airline can be found on the Skyscanner website.