One fan's speculation:
Misty - 15
Erika -16
Sabrina-17
Janine - 12
Whitney - 15
Jasmine -14
Clair - 18
Roxanne -19
Flannery - 17
Winona - 19
Gardenia -16
Maylene - 11
Fantina - 23
Candice - 16
Lenora - 26
Elesa - 19
Skyla - 16
Iris - 9
Different site asking Roxie's age (kind of like Yahoo Answers), but she's usually pinned between 13-15
Another site separate from the above only covers Unova
Cheren…16
Roxie…13
Burgh 28
Elesa 23
Clay 53
Skyla 23
Brycen 47
Drayden 67
Marlon 29
Grimsley 35
Caitlin 17
Shauntal 28
Marshal 38
Iris 10
Bianca…16
Prof. Juniper 34
Fennel 34
Alder 65
N・・・18
Colress 27
Ghetsis 63
Ingo and Emmet 30
Benga 16
Cilan, Cress and Chili 18
Lenora 34
None of this is canon, I even disagree with a few of them but it's just interesting to see where Japanese fans ballpark it.
