Neil Patrick Harris fans point out sad detail after seeing new photo of twins

Neil Patrick Harris fans have been left stunned by a new photo of the star's twin children.

How I Met Your Mother star Harris was joined by his husband, David Burtka, and their children, Gideon and Harper Grace, on the red carpet for the opening night of The Lost Boys on Broadway on Sunday (April 26).

The couple married in 2014, after first going public about their romance in 2007.

Gideon and Harper were born in 2010 after being carried by a surrogate and conceived using an anonymous egg donor and sperm from both Harris and Burtka.

Previously explaining how the process worked, Harris explained: "We inserted one of my sperm and one of David's sperm into two eggs with the hope that they would both take, just because we both wanted to be dads biologically... Both took, miraculously."

The couple have opted to never find out which of their children are biologically theirs, with Harris simply saying: "I have no interest in [finding out]. We are their parents and I love them implicitly."

(Instagram/@nph)

The twins have only been seen occasionally as they've grown up, with the couple sharing occasional photos and cards on special occasions, including highly-anticipated Halloween and Christmas pictures.

The twins are now 15, and fans can't believe their eyes after seeing the new photo of the family, all clad in red and black to promote the new Broadway play.

A stunned commenter wrote: "I can't believe how grown they are now. I remember seeing them as babies in their annual Halloween photos."

As another shocked fan echoed: "Me too! I would have expected to see 5-year-olds."

The couple appeared on the red carpet with their twins, and fans couldn't believe their eyes (Manny Carabel/Getty Images)

One fan wrote sadly: "That was also my first impression. Like they're grown now??? This is reminding me I'm aging," along with crying emojis.

"Holy hell, time flies!" shared another.

This creative reply opted for denial as a coping strategy: "Oh, so actually his kids are 5 years old tops. Thank you very much. Put this photo away you're messing with my timeline."

Another said they were 'blown away' by 'how old the kids are now', while another joked: "Cute family. I must be elderly. I thought they were toddlers."

One commenter rued the passage of time: "Weren't their kids just babies?? Where does the time go!!"

The happy family attended the premiere of The Lost Boys on Broadway (John Nacion/FilmMagic)

"I could have sworn they were born yesterday" insisted another, while one marvelled: "Their kids are the same height as them now!"

Burtka recently took Harper along to the premiere of Wicked: For Good in New York, and fans were thrilled to see a picture of them together there.

The proud dad wrote on Instagram: "I had the best time at the most over the top premiere Monday night @wickedmovie and @lincolncenter. I’d tag my gorgeous date but she still doesn’t have instagram."

It seems that their fans aren't the only ones bewildered by how grown up the kids are now, as he added: "It’s hard to believe she is 15 years old. I am so proud of her, she is a dynamic kid #grateful."