They could have meaning to you as ages of yourself or others. 19 is the last year of teenage, 20 is the first non teenage year, and 21 is the first fully adult age. So it could be about coming of age.
Alternatively, because they're in series, I would say they might represent some sort of series in your life, steps or a set of things.
I seldom am able to interpret numbers. What is more important is the actions in the dream and your feelings as they occur.
Not much. Dreams are all about emotions, and if there was no emotional reaction to the numbers they were just taking up space.
I heard somewhere that those numbers mean that they are the winning lottery numbers.