I don't think there's a hard and fast rule, it's largely a cultural thing. A 14 year old girl from ancient Israel might have been ready for marriage, most 14 year old girls from the modern US would not be. There are a lot of things which aren't clearly stated anywhere, a lot is relative to how people are taught in society.
There is an underlying rule of "morality" (If you will) though - I would say it is wrong to marry a girl who is too young to handle it. The difference isn't that they had a different rule in the past, it's that they had a different society from our own with different standards for what a woman needed to know. (And different standards for when, and how, and what they were taught when young)