Refreshing or Rude: addressing an older woman as 小姐 太太，奶奶，大姐，阿姨，女士，and many other variations are commonly used to address older women. 小姐 is used to address young women. Would it be a complement or an insult to address a middle aged or elderly woman as "小姐"? I'm in Taiwan here, so we don't have to worry about people thinking we're calling them prostitutes.
Use 女士 with her family name is always safest. e.g. 李女士