For children aged 3-6 there are grace and courtesy lessons in a Montessori classroom. For children aged 0-3 grace and courtesy is modeled by the adults. This entails the parents to be using their manners with the children and other adults that may be around.

More Montessori Sensitive Periods:

Sensitive Period for Order (age 18 months to 2 years)
Sensitive Period for Language (birth to 6 years)
Sensitive Period for Movement (birth to 4 years)
Sensitive Period for Refinement of the Senses (birth to 5 years)
Sensitive Period for Weaning (5 to 6 months)
Sensitive Period for Numbers (4 to 5.5 years)
Sensitive Period for Manners and Courtesies (2 to 6 years)