] Like you, thanks to JA and GH I was under the impression that the age of marriage for women was early twenties or even late teens.

From all my reading I suspect that the marriage age had a lot to do with finances. Most people in the class of "gentlemen" were not wealthy, and wouldn't be able to afford to marry at an early age. A prime example being JA's parents, who weren't particularly young when they married. Her father married as soon as he was able to financially, which I think was about 30. Her mother, from a well thought of but not wealthy family, was of a similar age.