I also remember the scenes with them as kids. Several of them, in fact. Mostly season 1, before all the Harry scenes became ghostly dark passengers, and were actual flashbacks to Dexter as a child learning the code.
Despite age being stated on the show, the ages of the actors cast to play the young Dex and Deb are much stronger evidence to the contrary that they are very close in age. Almost as strong as the evidence that Harry brings Dexter home with him as a child (around 4 or so) after Dexter's mother is killed, and Deb is already born.