A digital citizen is anyone who has an online presence. The matter of being a "good" digital citizen, which is often what I feel we are truly discussing when we bring up the concept of digital citizenship, is someone who engages online in a respectful and responsible way such that they are caring for the greater online community that they are addressing.
Being a "good" digital citizen is one of the hardest things to instill in our youth. It is not a matter of intentional ill will, it seems to be a matter of not living in a time before the internet to understand its real scope and impact. Those that have grown up surrounded by technology 24/7 don't understand what it is like to have "privacy." Therefore, they feel that everything is public and should be public. Therefore, they do not second guess sharing opinions, videos, etc. without considering that their audience is not just those they intend to reach, but rather, the world!