There is no age to learn new things. Learning can be done by any means be it by experience, teaching, observing etc. Small children are more prone to learn new things by observing their surroundings, how people around them are reacting to certain situations.
Also the kids mimic their parents (how they behave with them). Thus it becomes very important for parents to make sure to behave politely and treat everyone with respect around them. By the age of three to four child starts to grasp things what they learn by observing others so it becomes very necessary to show the gesture of kindness, politeness and with affection.
Also this is the time when you teach your child the magical words like thank you, please, sorry. Teaching empathy at the earliest stage is the best thing you can do. Gradually this will lead them to learn how to treat other with respect and be patience with things.