In the snap, Kash was pictured in a booster seat and dressed in a grey fleece jacket and khaki pants.
Though the former Real Housewives of Atlanta star mentioned her sons age, many fans couldn't help but express shock at his stature for such a young boy.
The mother of six hilariously responded to a fan who left a message about the first grader’s appearance.
“Holy hell he looks 25!” the fan wrote with Zolciak-Biermann replying, “Oh your day is coming.”
Another fan commented, “He looks so grown lol,” to which the Bravo star said, “Crazy right!!”
“He does not look 6 , he’s going to be a big boy like his daddy,” another fan echoed.
“Holy cow he does look like he’s 14 or 15 yrs old!” a user wrote.
Another follower exclaimed, “Lord Kim I was about to say who is this grown man on booster seat. Wow. I think he’s gonna be tall.”
And yet another wrote, “He looks like a 15 year old in a booster! Omg.”
Zolciak-Biermann is also mother to Brielle, 21, Ariana, 17, Kash, 6, and twins Kaia and Kane, 5.