The Representation of the People Act 1928 extended the vote to all women over the age of 21, the same age as applied to men (whereas the The Representation of the People Act 1918 had only granted it to women over the age of 30). The change came about largely thanks to a vigorous campaign during which women protested against the implication that somehow they were less intellectually competent than men.
Clark, Robert. "UK women's suffrage extended to ages 21-30". The Literary Encyclopedia. First published 13 October 2012
[http://www.litencyc.com/php/stopics.php?rec=true&UID=19330, accessed 21 February 2017.]