The list of players below is by age, starting with the oldest. The youngest is 19. This includes 9 of the top 15 ranked players. It's time, IMHO, to stop thinking of these players as 'up-and-coming'. They're here and a lot of them have already had some accomplishments of note.
04-13-79 Shaughnessy 2 titles
07-05-79 Mauresmo GS final
06-17-80 Williams, V GS 4
09-30-80 Hingis GS 5
12-15-80 Stevenson 0 titles
06-07-81 Kournikova 0 titles
07-08-81 Myskina 1 titles
09-17-81 Williams, S GS 3
10-15-81 Dementieva 0 titles
01-07-82 Tulyaganova 3 titles
06-01-82 Henin GS final, Tier I win
09-19-82 Danilidou 1 title
03-09-83 Bedanova 1 title
04-12-83 Dokic Tier I win (2)
04-23-83 Hantuchova Tier I
06-08-83 Clijsters GS final
The downside for a lot of them is, there's been so much accomplishment within the group. Dementieva has THIRTEEN players around her age who've had better careers to date. That's a lot. She may improve enough to transcend half a dozen of them. But ALL?
More to the point, the physical maturation of most of these players is complete. Or rather, the physical maturation of ALL of them is 95% complete. And most of them (Stevenson being a notable exception) don't let themselves get fat. THey pretty much are what they will be, physically.
We've seen how fast they all are.
We've seen how strong they all are.
We've seen all the shots they'll have without a great deal of off-season work.
At least half these players have the physical skills to win a GS title.
Only 3 of them have proven to also have the right mental/emotional mix to be GS champions.
Venus got her first GS title at 20. Serena got her first at 17, Martina at 16. Most players who win more than one GS title, win one by the time they are 20. (Martina Navratilova, however, won her first at 23.) So, I think it's fair to say that we've seen these players at their PHYSICAL best. We know what they're working with physically now.
Alexandra isn't going to to suddenly develop Justine-like speed.
Anastasia isn't going to develop Dementieva's forehand.
What we don't know is, who has the heart, the desire, the intellectual capacity, the emotional control and the mental focus to make themselves a GS champ?
But at this point, I think we can take a WYSIWIG approach to analyzing these sixteen players. Age isn't going to make any of them better. Training will. Emotional maturation might. Learning how to build points will help those that don't know how yet, if they bother to learn. But physical maturation won't. For these sixteen, what-you-see-is-what-you-get.