I have no problem at all ignoring as a willing suspension of disbelief but I do think they have set up enough hints that Henry is special that they could make him the only one who ages in NL. Now if Dylan Schmid also looks a lot older after the summer break… well then we just have to fall back on suspension of disbelief. But it looked like he had already had quite a growth spurt before he filmed the last two episodes last year, so he may not have changed as much over the break.
This reminds me of the stories of how many people in the studio wanted to fire the 13 year old Annette Funicello from The Mickey Mouse Club when it became quite obvious that she was hitting puberty. I heard she was always grateful that Walt insisted on keeping her on the show.