Most women do find it inappropriate and rude for someone to ask them their age.
But my mom is an exception as she doesn't find it rude or inappropriate to ask her age.
She doesn't get why it would be inappropriate but I've heard it has to do with the old days and how women then didn't like to let their age out and be considered over the hill or something like that.
I'm not entirely sure why it's considered inappropriate but that's what I've read and remembered from before.
If you're dating it's not a good thing especially on the first date to ask the woman her age but you may try and ask her later.