I agree especially now, that more girls are developing a power game, which takes longer to develop. Lindsay, Venus, Serena, and Jennifer were all pretty late bloomers. But since there break throughs one of them is usually a favorite for a title.
I think people expect a win around 17 or 18 simple because tennis players do not have a long shelf life. They usually can only play into their early 30's, of course there are exceptions. But I personally think that now most girls will start winning big titles at around 20-23.
Martina Hingis was one of the last players to do so well at such a young age. I'm not saying that another player won't do the same, but it's seeming less likely.