What is the average height for a 40year old woman ?

Average height: Height is. very strongly genetic. So if you are asking what should your average height be at age 51F you need to do your family tree for height in women at age 51 and then average that. That is average in your family. This assumes that everyone in your family is truly descended from the tree.

5'4" in US: Women reach their full height in their teens and maintain that height as adults (though the elderly may shrink an inch or so as their spinal discs compress). The average adult woman in the US is 5'4" but this can vary with your ethnicity. For example, average in Guatemala is 4'10" while average in Austria is 5'6" See http://www.averageheight.co/average-female-height-by-country