Mars is definitely over 18. Whitney is probably 14 or 15. I think Candice and Roxanne are over 18 (how else did they become teachers in the anime?) Misty is probably just over 18. Janine, Iris, May, and Dawn are the only ones that are not at least 13. Maylene I'd say is 15
I am 99.9% sure Misty is not 18 or over, I am sure it has been proven she is much younger, I would place her about 12. And some may argue this next point but it was so obvious Misty liked Ash, do you really think they would make Misty old enough to like Ash and be sent to prison?, In the beginning Ash was ten, you are trying to say Misty is probably over 18. (Brock is aleast 16/17 so he's not as disturbing when he flirts with the older women, who for obvious reasons don't even return it), and I am sure Brock has been made out to be older then the two.
Anyways I don't really think any of the ones below 18 are that great looking, especially Dawn, May, Iris, Misty etc, it's just too weird to think of them that way. I think however Flannery, Lucy and Gardenia are pretty hot (And if I were to guess. Gardenia 18-20 with a very weird fetish. Flannery 25 and Lucy bordering 30, I think Cynthia is pretty much 30 too).
I actually could list all the ages I think the characters all are at, but probably best for another thread.