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.
Harrison Ford was only 12 years younger than Sean Connery who played his father in The Last Crusade.
Kate Mara played Heath Ledger's daughter in Brokeback Mountain when he was only 5 years older than her.
What matters is what they look like, and how it is portrayed. The Morgan siblings are clearly portrayed in the first couple seasons as being close in age; likely within a few years. As those were the only seasons worth a damn, I'll stick to that.