Eh, I know of plenty O couples whom have a 10 to 20 year age difference AND first met when one of the parties was almost out of high school. Keyword being MET.
With that being said, my issue isn't the age difference: My issue is that Heechul is sus. I've always gotten a strange vibe off dude, way before his relationship with Momo.
Don't be shocked if Heechul ends up getting exposed before it's all said and done. I don't wish that on anyone but I just can't shake this vibe I get from him....