different cultures have different point of view about getting married.
as liberated as a girl can be nowadays, I think they can choose what's good for them, or what could a marriage brings in their lives. however, like for instance the situation here in our country, most of those (girls) who got married at early age, do not attain the fullest of their lives and sometimes, end up with having an 'unhappy family'. Although the marrying-age for a girl starts at 18, it does not necessarily mean that you can 'tie a knot' right away.
like what jura said, we can't specify the exact age of a girl to get married. after all, it's not the age that really counts, but until the fact that a girl will be mature enough to handle a 'new' responsibilities.
:)