I was under the impression that these movies have been getting filmed for about 3-4 years already.. and they'd start releasing a new Avatar movie once a year. But they're still casting roles!? Wow. At this rate I won't even expect Avatar 2 to come out in 2020. Also, I haven't seen this actor in much besides Fear the Walking Dead. While hes probably the best actor in that show (thats not saying much honestly), I don't see him doing well as a lead in a major movie franchise like this.
 
Source: https://www.polygon.com/2017/5/9/15594872/fear-the-walking-dead-avatar

http://www.icv2.com/articles/news/17950.html
Seeing as it is 70 years later, and the girl will probably be in her early teens, I'm assuming Ang must have died around 60 years old? And if I remember right, the avatar cycle should make the next avatar from the water tribe.