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.
Michael C. Hall is 42; Jennifer Carpenter is 33.
Reread the quote. I said YOUNG Dex and Deb. I'm not talking about Michael C Hall or Jennifer Carpenter, nor am I talking about adult Dexter and adult Debra... I'm talking about the various actors cast to portray the two characters as CHILDREN. I do not dispute that Dexter's character's birthdate is given in the show as 1971 and Debra's is given as 1979, in fact I point out that it is a continuity error because of the much more bountiful evidence that this is not the case.
I finally broke down and looked up some specific childhood events that dispute the claim that Dexter is 8 years older than Debra:
Dexter is formally adopted by the Morgan's at age 7 after living with them since age 3. A young Debra asks Harry what happened to Dexter's real parents.
Doris Morgan (Dexter and Deb's mother) dies when Deb is 12 and Dexter is 16.
Harry Morgan is in the hospital for several months and dies when Deb is 16 and Dexter is between 20-21 (he has a prolonged sickness in the hospital, and the nurse who has been poisoning him is Dexter's first kill). This is later contradicted by the current season, where Harry Morgan claims he witnessed Dexter's first kill and is mortified, implying that he takes his own life shortly thereafter.
Other evidence shows Dexter as being in the class of 1991 (his 20th high school reunion being in 2011), which disputes the claim that he was born in 1971 because then he'd be 20 as he graduates high school, which seems unlikely for someone who had good grades and was at the top of his college class in medical school.