So, there will be times when you will obviously not be able to walk or talk or act like the 20 year-old you. That is when one of the most important skills of the actor is called upon. The skill of observation.
Among your family and friends there are bound to be people of the various ages you will be called on to play. But do you just go out and imitate the first 11 or 14 or 40 year-old you come across? Obviously not. They all have to have similarities that will demonstrate to the audience that they are one and the same person at different stages in their life.
You will need to get out and about, even more if you haven’t done so up til now. You will need to observe people. Watch how an 11 year old walks, how a teenager talks, how someone of 40 behaves. You can do that anywhere you are – on the street, at a family gathering, in a supermarket, at a party. For a time, if you are at a party for instance, drop out of things for a while, give up the girl or boy you had your eye on, leave your friends for just 20 minutes, and observe. Wherever it is you are, just watch people, note how they walk, talk, behave, present themselves, and then click! Imagine you are a camera and your eyes are the shutter. When you see or hear something interesting, just imitate the action of the camera. Click your eyes, take the photo, and consign it to your memory bank. When you need to make a withdrawal from the bank, you select the right image and bring it to life.
All this requires a conscious effort. A successful actor is as much an observer as a performer. From your memory will spring characters with ways of talking, or walking, breathing, smiling or crying that will make them alive to the audience. The real people you saw on the street, at the footy, at a family celebration or a party will once again become real, when you recreate them on the stage.