I'm going to say this now. As Rosai is a "late" Avatar, she's already in her early 20s when she's discovered to be the Avatar. Those who are making or want to make characters as her peers, aim for the age range of 19-25. Most roles can ignore this, but [love interest] and those aiming for "Team Avatar" status should probably be within that range.

You can be an older member of Team Avatar (Lin Beifong or Tenzin, as examples), but if you want to be the "Mako, Bo Lin and Asami"-type peers, you have a range of ages.