I found this in a quick search, but it only refers to those who were allowed to go to the fronts.
"At first the military authorities were unwilling to accept VADs on the front-line. However, this restriction was removed in 1915 and women volunteers over the age of twenty-three and with more than three months experience, were allowed to go to the Western Front, Mesopotamia and Gallipoli. Later VADs were sent to the Eastern Front."
Source -
https://spartacus-educational.com/FWWnurses.htm
Also found this,
"Volunteers for the Nursing Service of the VAD had to be at least 21 years old for home service and 23 for service overseas and of a maximum age of 42 for those overseas and 48 at home."
Source -
https://www.militaria-history.co.uk/...id-detachment/
Terry