I agree with MoonPeachBlossom, this is Elsa at twelve/thirteen, since she still has the band in her hair. Elsa at 18 had her hair done in that same twist her mom had, so doesn't have a band over the top, but has a blue ribbon woven through to keep it in place.
BUT that's just the title. The work itself is fantastic, especially the hints of frost on that lovely blue dress. We saw her really distressed in the movie, so this seems to be a happy moment. Nice to see, especially since we never see her happy since the accident until she's on the North Mountain.