So basically is a shoujo + smut manga. Not yaoi. I read this long time ago can't remember the title. It's about this girl taking care this young boy which is her neighbor. She bring him to this lake and this kid changed to handsome teenager guy. (づ ̄ ³ ̄)づ they end up sleep together when the boy still big..
Not sure, but seemes similar
http://www.mangago.me/read-manga/mayonaka_wa_dame_yo/
This story is written beautifully. I bawled my eyes out from beginning till the end ╥﹏╥