Aren't twins supposed to be the same age?

"The Unborn" is being released to DVD or something. I know this because there are tons of commercials out for it. When the movie first came out I did not get around to seeing it. It looked pretty bad, and by pretty bad, I mean awful.

Well, the commercials for the DVD release don't make it look any better. The Megan Fox look alike (Odette Yustman - I just imdbed her) is being haunted by some small little boy. In the previews a man, who I assume is her father, informs her that she originally had a twin who died. The boy haunting her is the twin.

Here is my beef with this movie. If the boy is her twin how come he is like six years old when she is clearly 18 in movie years, probably closer to 20 something in real life. Wouldn't her twin be the same age? Why would he take the form of someone so much younger? That is such a huge plot hole to an already bad looking movie. Geez Holywood, what were you thinking?