Among the rare series with an evergreen, timeless appeal, “Avatar: The Last Airbender” is particularly special. Despite having originally aired on Nickelodeon, and despite being marketed as a children’s title, Michael Dante DiMartino and Bryan Konietzko created an epic with an appeal that spans generations. Fifteen years after its premiere, all 61 episodes of the animated series arrived on Netflix in May without much fanfare, save for the fandom sounding the call and announcing its availability. Since that day – May 15, to be precise  -– the adventures of Aang, Sokka, Toph, Katara, and their enemy Zuko (and his gentle uncle Iroh) have held on to a spot among the streaming service’s Top 10 most popular titles, holding the No. 1 slot in the earliest days of its tenure. Ordinarily a series’ or film’s ranking on Netfix isn’t worth noting unless its popularity adds context to the discussion, and I don’t have demographic data to break down the ages or backgrounds of who is watching. In this case, though, the sustained popularity of “Avatar” gives me …