Well, determining Claymore ages isn't really an exact science or anything
Unlike many other animes, the voices for most of the Claymores are a little more deeper and more warrior like. Much different from the many female voices you hear in most other anime series.
I agree that you can't always go by personality to determine someone's age.
As for Clare, I've always had the feeling that she was a pretty young Claymore. She doesn't know about many things and has to be told of them. This is probably due to the manga-ka introducing storylines and aspects of the Claymore world to the reader, but he made it seem like Clare actually knew little about various things. Like offensive and defensive types of Claymores. Or what Voracious Eaters really are. etc. etc.