Any age is a good age. One is never too young or too old to learn good manners. Start how you mean to go on and start today!
I was recently on an international flight from Europe to Vancouver. A relatively long haul even for the best seasoned traveller. In the row in front of me sat a young couple and their adorable 18 month old son whom, unfortunately, had the most awful cough and cold. While my heart went out to the poor little chap who was coughing and sneezing away into thin air I also felt sorry for my fellow travellers, especially those prone to respiratory infections.
It is just as easy to teach children the good manners of coughing and sneezing into their upper arm as it is to demonstrate playing Patty-Cake by clapping your hands. Make a game of it and pretend to cough and sneeze into your upper arm. Children love games and learn through role play. Eventually, through practise, they will exercise good manners naturally – without prompting!