@Moodyz Chizu is more than "a bit" older in this bonus chapter, she's significantly older. I'm pretty sure the implication is that this bonus chapter (and by extension the entirety of No Youna) takes place ~9-10 years after Sorenari ni Shinken Nandesu, since Chizu was in Kindergarten for that series (4-5 yrs old) and she's now in middle school.