Marianne is in my mind no different than the average 17 year old of her time, or of our time, in her view of age. In fact, she would fit right into our youth-obsessed culture, as promoted by the media, particularly Hollywood. However, since young women of that period frequently married older men, and young men sometimes even married much older women for their fortunes, it may be JA's way of making us aware of how Marianne's temperment was a reflection of the cult of "sensibility" and romanticism that was so popular at the time. We (modern day Americans) are also creatures of that sensibility, in that we look at love/marriage as primarily hormonal, while many other cultures (past & present) view marriage as primarily economic or dynastic, with hormones a distant third (or worse!) in the list of reasons to wed/love.