Yeah, that part kind of upset me. I yelled "she's 8!" at the game when he asked, and was shocked when he yelled "wrong, she turned 9 six days ago".
But who knows, maybe Clem forgot. I'm sure they didn't keep a calendar around in the zombie apocalypse. Campman was probably trying to be "fatherly" to Clem and ask when her birthday was, she tells him the date, and he's like "that was a few days ago!"