If she is a teacher or someone great. Yes you can. People call my grandma 许先生，cuz she used to be a good teacher.

先生 for man, 女士/小姐/ for woman. This is general rule. But if the woman made great contribution in science,literature,art,politics,etc. She also can be said 先生 by authority before.Only a few women.It's just like reward. this is not general rule, sometime it's political.

现在的中文先生一般指男性，1949年之前，很多有知识的女性也被称作先生

Only for men. For women either 小姐 or 女士 is used.

Only to the man. Woman is called 女士. It's just like the words sir and madam. We can add friends and talk to each other, learn from each other.