I just started watching the second season of the show so if my question will be answered later just let me know without any spoilers please. Now my actual question: Henry was born when Emma was 18. She gave him up for adoption at Storybrook. People in Storybrook have had the same age for 28 years. I suppose for the last 28 years they did not notice time passing and they did not create any new memories? (similar to the Memento guy?). But Henry was aging. He was taken to the village at age 0 and he found Emma when he was 10. Since all other characters do not age didn't anyone notice Henry changing classes every year while they stayed in the same class? Didn't anyone notice him growing up every year and them staying the same? If it's a memory thing (they forget at the end of the year or something like that) then wouldn't Regina expect that Henry would eventually realise that something is really wrong? (as he did at the start of the show)
In a few places, the curse is specifically mentioned as only effecting those 'not born in the land without magic'. Henry was not born in the Enchanted Forest, and not even his mother was affected by the curse, since she was transported to the 'land without magic' prior to the curse hitting.
As a result, Henry was an uncursed human living in a cursed town. The town was stagnant and unchanging, but Henry aged and grew as normal. Early in the show, Henry mentioned that everyone in the town existed in something of a haze and didn't perceive the passage of time (because time didn't pass in Storybrook), and so were probably sufficiently hazed out that they didn't notice Henry aging among them. Their minds would simply adapt to believe that of course he was in that grade, despite the fact that none of his peers moved up with him.
Naturally, this would be really, really freaking weird for Henry, and is not really addressed in the show. This is probably a contributing factor to his coming to believe in the curse and that everyone in town was fairy tale characters. After all, being the only person aging normally in a town of functional immortals would set off the weirdness alarm in just about anyone.
As for why Regina didn't think of this...well, um, the plot? It is canon that Regina adopted Henry because she was lonely and wanted something real and changing in her life...maybe that overrode her better judgement? She has also been shown to still have the ability to concoct potions, even without full access to her magic, maybe she thought that if it got too bad, she could use a memory potion to adjust Henry's memory to ignore the stagnancy of the town.
All in all, it probably wasn't too smart on Regina's part to not realize that Henry would notice that something was wrong. But she may have assumed that he wouldn't come to the right conclusion about what was going on. I mean, really...of all possible explanations, 'everyone in my town is a displaced fairy tale character' is pretty unusual, and probably wouldn't have happened at all if not for the mysterious storybook (which is covered in a bit more detail here)
(Half spoilers if you are really smart) Henry does age because he was born in the real world and because Emma (his mother) was not cursed (she escaped the curse). Regina, his adoptive mother, and the one who cast the curse, did notice he aged, so she figured that it would be safe to keep him because all of the fairytale characters did not age, and he wouldn't be against her. What she didn't know, was that Henry's real mother (Emma) was the savior, daughter of Prince Charming and Snow White. The other kids did notice him growing but just focused on the fact that they thought he was crazy thinking they were fictional characters