The earliest reference to this concept that I can find is in a 1996 speech by Tim Berners-Lee. He uses the term vaguely in connection with a mention of web content filtering schemes. In the US, one can think of 13 as the age of digital majority, since this is the age, per COPPA, that someone can sign up for a website without written parental permission.
Thinking in a broader sense, this concept touches on the question of at what age one ought to do the things that adults do online, like posting in public or having one's own site. Jessica Reingold suggests that given the tendency of adolescents to say and write things they regret later and the permanence of everything on the public Internet, this age migh be as old as 19 or 20.