What is the average weight for a 26 year old woman in stone?

It depends entirely on height. Most commonly the Body Mass Index is calculated for the individual, this uses and individual's height and weight data to indicate a BMI value.

A BMI with a value between 18.5 and 25 is generally considered to be within the normal range.

See the NHS BMI calculator.

http://www.nhsdirect.nhs.uk/magazine/interactive/bmi/index.aspx