Ichigo and anyone who goes to school with him (who isn't a Shinigami/Vizard) are underage. For the Shinigami, Vizard and Arrancar it gets trickier, IMO. Do you go by physical age, in which case Yachiru, Hitsugaya, Wonderwyce and Hiyori are all counted as underaged, or do you go by actual age, in which case all Arrancar created less than twenty years ago are underaged?

Most writers ignore the ages of the characters in smut fic anyway, either by reminding their readers of character ages or by putting up a disclaimer that says that all characters have been aged to match the age of consent in whatever state the site server is hosted in.