Huge difference between asking what age women are ready to settle down, and what age do women settle down at.
Here in the Midwest, most women think they are ready to settle down by their mid-twenties, but then a lot of them end up cheating on their husbands by the time they're about 30 or so because they realize they got married too quickly. I've seen it happen several times with friends of mine. It's unfortunate for everyone involved.
In reality, I think you're spot on with the assumption that on average, most women are probably ready to settle down by the time they're 30 or so, give or take a few years.
There obviously are exceptions and variables depending on where you live though, as well as family situations. If a 24 year old girl has 2 older sisters who are both happily married, she's going to be really feeling the pressure to settle down, because she hates going home for the holidays as a single girl with all these couples around in her parents' home.