Heard the author is keeping this 15+ for this season, wich is supposed to be the last seas...

Who said this is the last season?
Just check the afterward, in it the author says season two is rated that way which makes sense, it's gonna take time for them to get closer and mature. There's also a lot of plot waiting to be unraveled. Pretty sure the rating will.change by s3, but for now it makes perfect sense. Plus they are soo adorable as characters and the art is chef's kiss.