@
Ontheroad the age cutoff for saying "girls"
That's context dependent. Women of all ages in Norway frequently refer to each other as
jenter which the dictionary (Bokmålsordboka) says refers to female children, unmarried women, female servants or workers.
British women do the same.
A man should probably not address the same group of women as girls but as ladies unless he is very close to them.
In some formal contexts perhaps females makes sense.