They aren't though :) They're only adults in (some) the extra chapters, they haven't grown up and we haven't seen their birthdays(which i would assume, knowing the author we'd get to see them celebrate eachothers birthdays), we only got to see Jians birthday because he probably turned 16/17. Indicating that they are not infact adults :) I know it may seem like they are but even HeTians brother said he won't meddle in " kids fights ", there's so much more evidence in the fact that they are not of age yet compared to the guess of them being 18 :)
good sleep schedule? Not round here...