In case of emergency, the passengers sitting near the emergency doors have to carry out certain tasks, they are given instructions on how to check the situation outside the airplane and how to open the windows during an emergency. They receive this briefing beforehand from the flight attendants. These passengers are expected to be able to carry some weight cause they are expected to throw the windows out.
Also, anyone below the age of 15 is not allowed to seat next to an emergency door, the passenger must be 15 years or older. The passenger must be in good health, without disabilities and must not be in travelling with some one that needs special assistance in an emergency.