Age of consent actually varies from country to country, and from state to state in the US. I think a lot of people would be surprised to see what ages are actually leagal in many places.
Worldwide:
http://www.avert.org/aofconsent.htm
US (possibly more detailed):
http://www.coolnurse.com/consent.htm
http://www.actwin.com/eatonohio/gay/consent.htm
Most publishers don't seem to take this into account. I see a lot of submission guidelines that say "No pedophilia". I think what they really mean is no sex under the age of 18, though. In truth, pedophilia is specifically and legally defined as sex with a pre-pubescent child. Many people use the term inaccurately to describe underage sex, and many people don't actually know what the legal limits are, even in their own state.
I wish publishers were more specific about exactly waht they mean in their submission guidelines, but I suspect many of them are as clueless as the majority of people are.
If you read some of these guidelines they're pretty silly, for example the age of consent in Massachusetts is 16, although it's illegal if the person is a virgin, according to one web site (I haven't read the actual statute).
The law in my own state (Wisconsin) reads like this:
948.01 Definitions. In this chapter, the following words and phrases have the designated meanings unless the context of a specific section manifestly requires a different construction:
(1) “Child” means a person who has not attained the age of 18 years,
except that for purposes of prosecuting a person who is alleged to have violated a state or federal criminal law, “child” does not include a person who has attained the age of 17 years.
Now, doesn't that make it sound like if you have sex with a 17 year old that they aren't actually considered a child after all? I suppose you could charge them with a misdemeanor. Note that federal statutes cover using phones and mail for purposes of soliciting sex, especially across state lines, but they only apply if the act is criminal in the state in which it takes place.
Ages of consent used to be lower, because people used to marry much younger in order to have children as young as possible, to help work the farms. Society has changed though, and the right to marry has been taken away from those people who have been disenfranchised by the constitutional amendment that set the voting age at 18. By federal law all that was required to vote was a 6th grade education, although most states restricted it further.
There are organization dedicated to lowering the voting age to 16, here is one of their web sites:
http://www.youthrights.org/votingage.php I find a lot of their arguments convincing.