@Michael Loucks
It would depend on where you were, as well. In Ohio, the age of consent was 16, so 18/16 (or 38/16) would have been legal. In the early 80s, if the girl was 15 and the guy college age or younger, it was generally ignored, or plead out as misdemeanor 'contributing to the delinquency of a minor'.
When makes a bigger difference. From what I've read, in the late 19th Century US, the median age of consent was 10.
However, the most important consideration is not where the story is set, or when, but when and where it is being published.
When posting stories on-line involving under-age sex, you need to be aware of where the site's servers are (SOL is in Canada) and what the law is there. If the country where the servers are located decides to bring charges against the author, they aren't going to care what the age of consent was when/where the story was set.
Among Western Democracies, the US is unique in virtual child porn being legal.
And in the US, some writers of child porn stories have been prosecuted not under child porn laws, but generic obscenity laws. Though, in all the cases of successful prosecutions I am aware of the stories were alleged to have included not just under age sex, but violent rape and torture of prepubescent children.