Most airlines will accommodate passengers who have special needs, the more notice you give them, the better your journey is likely to be. If you have a flying phobia or have a broken leg, the airline will be able to make some kind of arrangements for you. When you confirm your booking, it is a good idea to remind them of your needs and again when you check in at the airport.

Passengers in wheelchairs are catered for, with most major airports providing ramps for wheelchair access and escorts from the check-in desk to the airplane. Aircraft toilets are likely to cause problems, so ask for advice from the airline staff and a doctor before booking your ticket.

Blind passengers travelling with their guide dogs should check with the airline about any special arrangements. Guide dogs will have to travel in a pressurized baggage compartment with other animals, away from their owners. Sometimes small guide dogs will be allowed to travel with their owners in the cabin - If you are unsure ask.
Guide dogs aren't subject to quarantine as long as a certificate of vaccination for rabies can be produced.

Deaf and hearing-impaired passengers can ask cabin staff to write down information for them - airport and in-flight announcements etc.

Children between the ages of 2 and 12 have their own baggage allowance and seat prices for them are either half price or two thirds of the full fare.
Children under 2 years old travel for free on most airlines if they don't occupy a seat, although some may charge up to 10% of the standard fee. There are no baggage allowances for children under the age of two. The airline can provide cots for babies up to 22lb as long as you notify them well in advance.

Generally speaking the airlines will accommodate passengers needs - the key is to notify them in advance of any queries or requests you may have, this will have the benefit of making your journey as comfortable and pleasurable as possible.