Fans of actor Neil Patrick Harris are convinced that one of his children bears a stronger resemblance to his husband, David Burtka, than the other.

This revelation follows a photo shared by the star on social media, which left users feeling 'really old'.

Harris, 52, and Burtka, 50, are the proud parents of 14-year-old twins Gideon Scott Burtka-Harris and Harper Grace Burtka-Harris.

The twins were welcomed into the world in October 2010 through a surrogate. At that time, two eggs were implanted in the surrogate's womb—one fertilized by Harris’ sperm and the other by Burtka’s.

They’ve consistently stated they have ‘no interest in [finding out]’ whose child is biologically whose.

Over the years, the well-known couple has shared intimate family moments with fans, including the heartbreaking news of their dog passing away.

The family's latest post has gone viral (Instagram/@nph)

Recently, the American couple shared another family milestone on Instagram: the twins’ impending first day of high school.

“The kids are starting high school this year (how did that happen??), and they are finally allowed to move on from their Chromebook anchors to the world of MacBooks. Huzzah!” the How I Met Your Mother star wrote.

The caption accompanied a picture of Harper and Gideon smiling at each other over grey and black Apple laptops.

The comments section was abuzz, with many Instagram users expressing disbelief that Harris’ kids are old enough to attend high school this September.

“Hey, I’m pretty sure they were just toddlers a few weeks ago, so there’s no way,” one fan commented.

Another responded: “This looks like the opening scene for a new Disney Channel show!”

Neil Patrick Harris often includes his family in Instagram updates (Instagram/@nph)

“And I was just about to start high school when they were born. How time flies,” someone else remarked.

A fourth Instagram user, however, has used the image to comment on Harper and Gideon’s parentage.

“No doubt whatsoever whose kid is whose biologically lol,” they replied.

While the user didn’t specify whose child they thought belonged to whom, fans have previously speculated.

In older photos of the twins, people have commented that ‘Gideon looks more like David’, while his sibling ‘looks like NPH’.

Another user added: “I think [Harper’s] Neil’s & [Gideon’s] David’s lol.”

Someone else commented: “I agree! The boy looks just like David!!”

The twins were born via a surrogate in October 2010 (Instagram/@nph)

Meanwhile, one fan wrote: “Aw, they’re adults now. I knew Gideon would look like you, but wow. Beautiful preteen adults.”

Others, however, are less concerned about whose child is biologically whose, with some stating: “How is it that they honestly look like both of you?! They're gorgeous.”

Another said: “These two resemble both of you so much, it’s impossible to tell who’s biologically related to whom. That’s so cool.”

Despite not knowing whose child is whose, Harris has previously admitted that he and his spouse have suspicions based on their kids’ behaviors.

Neither of the pair has ever voiced these opinions, so we’ll just have to keep guessing for now!