Dead Twins Age Slower

So after seeing the previews for the movie The Unborn a thousand times, I finally noticed the inconsistency in ages between characters. Now, all teh information may not be available from a 30 sec spot, but it appears as though the main character (a college age girl whose twin brother died in utero) is being haunted by her unborn brother, who appears to be about 10 years old. Now, i could understand if it was an infant - being unborn and all he never aged - although a ghost baby is really not that threatening, and I can understand if it was a college aged ghost, but what's with the age discrepancy? Creepy kids are definitely great for scary movies (still totally freaked out by The Ring girl), but seriuosly, how are the going to explain this? Maybe you age in half time if you are stuck in the netherworld or something. Anyway, not that I was going to see the movie regardless, but it is definitely off my list now that I've brought this stupid problem to my attention.