But Zuko is 16 in the series and she is his younger sister so she is 15 or younger. I think Katara was 14, while Aang and Toph were 12 and finally Soka who was 15. I think Zuko is the oldest of team Avatar, not counting the 100 years of Aang being frozen. But I gotta agree, she does looked 17 - 18 in the series.
The cycle won't be broken yet, maybe when the next Air bending avatar's turn comes, but Raava cannot die, she is fused with the sould of the avatar and will reincarnate with the next one. Reason why Vaatu separeted Raava from Korra instead of just killing Korra. And I think that if Raava starts to grow from Vaatu, it will have to wait until Akira dies, so that two new avatars can be born since Vaatu no longer has a body of his own, to bad for Raava, since Akira doesn't age and to kill him it will require the whole world probably... maybe the next iteration of both Raava and Vaatu will be twins, would be a nice spin-off.
First time hearing that Tiamat is not referenced as a dragon. Like to me it is the opposite, I always hear from her in different games, like Final Fantasy series for example, she is always reffered as the Dragon Mother everywhere I see. I don't do research and stuff just saying my experience from playing games and reading novels, even a quick google search almost all the images are of five headed dragons. I haven't read the mythos of her though.
Aslong as she is weaker than acnologia she has to be away from them to keep him away from her loved ones. Even if she gets away, Acnologia can smell Dragons, reason why Igneel and company fused with their adopted human children, along the dragon sickness. If she is alone or with super strong mages Agnologia can't kill her, not easily atleast, since alone she can teleport away and not worry about her family. Don't know how she survived 400 years of him hunting and eating dragons though...