Love has nothing to do with bodies.
You said what I've thought. If they put JJ and Mew or Mint, and or Ann and Ken or Sam, it will be weird for the story line.Personally it make sense to cast James in here because he is supposed to play an 18 years old and many of you have said that James has a baby face, so it would only make sense to cast someone who does look younger than their age. As for Anne, because the storyline is saying she acting as someone who is 35, so she's not supposed to look young or almost age like James. This is a aged gap drama, it wouldn't be an aged gap drama if both leads doesn't physically look their part. I would understand if the storyline wasn't an aged gap but because it is, it doesn't make sense to cast Anne and Sam together as lead.
Also, both James and Anne are great actor and actress, they will make the chemistry possible for their viewers. Just from the fitting pictures alone I can already sense the chemistry between them.