When I was a travel agent I got a lot of enquiries from parents who assume that their 17 year old is classed as a child. Generally the maximum age for a child is 12 years old on the date of return. So a 13th birthday party while away could prove expensive.
An infant is classed as under the age of 2 years on the date on return. They do not usually have their own seats, but if you request a bulkhead seat, you will be given a skycot for use after take-off. If you don’t arrange for a skycot, then the infant will have to be on someone’s lap for the whole journey. On a long haul flight it may be worth paying for a seat, just to make life easier during meals. Check to see if a car seat can be used to keep them securely fastened.
An infant may not have any checked baggage allowance or in-flight meals. Check the latest security information to ensure that you can take baby food on-board (including milk or formula).
A child can be under 16 years old on the date of return on charter airlines such as Thomsonfly and no frills such as Ryanair and Easyjet. But with BA and Virgin Atlantic it is under the age of 12.
Buggies and pushchairs can use used up to embarkation, but they will then be placed in the hold. A label would have been attached at check-in. These are usually (but not always) in addition to any luggage allowance. Check for the procedure for collecting them. If you have to get them from luggage reclaim you may be in for a long walk while carrying your child.