I'd say 18-24.
My reasoning being feminism reduces women to their looks. Under more traditional cultures other attributes are fostered in women such as motherhood, homemaking and spirituality. So uglier women have a greater chance of being accepted under traditional cultures because in spite of their ugliness they'll bring other offerings to the table. It's funny how feminism claims to be against objectifying women, yet reduces women to sex objects.
On the flip side I think the older a woman gets in a feminist society, the less value she'll garner. Women whom are past past age thirty in a looks driven society are already super paranoid about looking old and wrinkly. This is why young women should be tamed into marriage. Young women are at the height of their beauty and they think their beauty will last forever. Only when a woman is young does she have one shot at capturing a man with her beauty who'll then stick with her even when she turns old.
I suppose this is why most of the feminists I see online are the 18-24 age bracket.