The age thing might not have been too bad; I don't know, I've never seen the manga, but high school and college are close enough that they could only be like two or three years apart.
Also, while some manga and anime certainly oversexualize underage characters, in general that sort of behavior is looked down upon, both in the real world and in the fictitious ones animes and mangas portray.
As for suggestive content involving a high schooler, Chinese law states that fourteen to sixteen is the age of consent, so this is what they are used to. While it is shocking, to be sure, remember that even the Bible states only:
"If anyone is worried that he might not be acting honorably toward the virgin he is engaged to, and if his passions are too strong and he feels he ought to marry, he should do as he wants. He is not sinning. They should get married." - 1 Corinthians, 7:36
Different people mature at different ages. God only required that they have fully matured into an adult. Granted, I don't think anyone is a mature adult at fourteen, and likewise feel anyone below eighteen is off-limits, but I'm not Chinese. Keep in mind, in Asian countries, adult behavior like working a job and such are encouraged very young, and so teenagers there are stuck in a strange limbo between childhood and adulthood, even more so than most teens.
All this to say, I don't think the creators of the manga were intentionally sexualizing minors, at least not in their own eyes. So don't feel guilty; for one, you stopped when you realized what you were getting into, so that's not an issue. And for two, most anime and manga characters are portrayed as fully matured adults, even in their high school years. (Especially the females.) Granted, there are numerous animes and mangas which have sexualized minors, but from what you tell me, that wasn't the intent of this one, nor was it your intent to look at someone that young in such a way. So don't worry. Sin will always come your way; you shouldn't feel guilty about getting into something and finding out later it has some sinful stuff, so long as you leave as soon as you know what's wrong.
