Kourtney Kardashian fans shocked over son Mason Disick's age as he celebrates birthday

Kourtney Kardashian fans are officially feeling old after discovering just how grown-up her eldest son Mason Disick is now.

Reality TV star Kourtney’s firstborn has just celebrated his birthday, and the milestone has sent Kardashian followers into a full nostalgia spiral.

Mason became an unexpected breakout star when he arrived in the season four finale of Keeping Up With The Kardashians. Viewers quite literally watched his birth on the show, making it especially surreal for longtime fans to realise he’s now not far off from becoming an adult.

The teenager is the eldest child of Kourtney Kardashian, 45, and her ex Scott Disick, 41. The former couple, who dated on and off between 2006 and 2015, also share daughter Penelope, 11, and son Reign, nine.

The revelation came via a sweet Instagram post from Mason’s grandmother, Kris Jenner, who marked the occasion by celebrating two birthdays at once.

Sharing a tribute to both Mason and his younger brother Reign, Kris wrote: “Happy birthday to our birthday twins, my amazing grandsons, Mason and Reign!”

Speaking of the eldest brother, she continued: “Mason, I can’t believe you are 16!!! Watching you grow into such a kind, thoughtful, creative, and confident young man has been one of the greatest joys of my life and I’m so proud of you.”

While the post itself was heartwarming, it was Mason’s age that truly stopped fans in their tracks.

One follower commented: “He’s 16! omg i remember watching his birth.”

Another echoed: “16?? I remember when he was born.”

And a third summed up the collective disbelief, writing: “16, wow, feels like it was just yesterday when it was prime keeping up and Kylie threw her 16th birthday party Alice in wonderland.”

Although Mason now keeps a much lower public profile than he did as a baby, he recently reminded fans just how fast time flies when he made his Instagram debut earlier this year. At the time, fans were similarly stunned to learn he was already a teenager.

"Wdym Mason Disick has an IG page, I watched his birth. We’re getting old!" one fan tweeted.

Another went on: "Mason Disick is now on Instagram - yes, we’re all old asf."

These days, Mason has stepped back from the spotlight, making only occasional cameos and choosing privacy over frequent reality TV appearances.

According to E! News, Kourtney previously explained that he 'doesn’t want any part of it' when it comes to social media and public exposure, and he has rarely appeared on The Kardashians or family reality specials as he’s gotten older.