When I was little, say adolescent, my Mom used to make us hold doors for her. We'd be going to the department store, for example, and she would just stop dead in her tracks in front of the door. At first we were like, "what?" but then we'd figure out that she expected us to open it for her.
Good job on your Mom! I started when mine were still small but old enough to hold a door open (probably 5ish). At first I would just suggest to them that they open the door for whoever the person was. They always got very positive feedback, which they liked, so they continued.
susie