He's clearly aged more than 2 - 3 years during the show. Of course that's because Chandler Riggs has aged 8 years during filming and not the character aging that same 8 years.

Doesn't matter: my point is that whether it's 3 - 4 years (using your timeline, rounding up) or 5 - 10 years (using Carl's development as a guide), no one develops a dialect in that timeline unless it's intentional.

If I wanted to be completely fair I'd probably lay the blame for that at Robert Kirkman's feet, assuming the heapsters are also in the comic and talk the same way. Other critiques of Gimple stand, though.