Billie Eilish's 21st Birthday Photo With Jesse Rutherford Sparks Discussion

Billie Eilish celebrated turning 21 by posting a loved up photo with her 31-year-old boyfriend, Jesse Rutherford.

The "Bad Guy" singer threw a Christmas-themed bash for her milestone birthday with some big name guests, including Justin and Hailey Bieber.

Eilish dressed in a figure-hugging red Santa dress, matching bolero and finished the look by styling her black hair in a vintage wave.

Posting a series of photos from her big night to Instagram, one shot in particular caught the attention of fans.

In the pic, Eilish is facing her older boyfriend and they are leaning into each other, smiling as though they're about to kiss. Behind them are gold balloons which read, "Happy 21st Birthday."

The couple have come under fire for their age gap with many people heading to Twitter to debate the relationship.

"Directing criticism at her is sooo strange to me like she only just turned 21 and idolised that man from when she was a kid!! the one getting the hate should be mr ugly," tweeted one person.

Another added: "Man in his thirties celebrating his girlfriend gaining the ability to legally drink."

And a third wrote: "He's going to the deepest darkest part of hell, like how you fumble devon and pop out with a 20 year old at your gargantuan age."

Eilish and Rutherford—the lead singer in the band The Neighbourhood— have previously addressed the criticism of their age gap, dressing as a baby and an elderly man.

The pair were first spotted kissing in October outside a Los Angeles restaurant. They later when Instagram official when Eilish later posted a photo of Rutherford to her account.

The couple then made their red carpet debut in November at the LACMA Art + Film Gala, wearing matching Gucci outfits and wrapped themselves in a quilt from the Italian brand.

Later that month, Eilish opened up on the relationship in an interview with Vanity Fair.

"I just am really inspired by this person, and, you know, he's inspired by me," she said.

"It's really cool, and I'm really excited and I'm really happy about it."

Eilish then responded to the backlash over their age difference: "Say what you need to say, but, like, I am in control right now. I did this. I managed to get my life to a point where I not only was known by a person that I thought was the hottest f****** f***** alive, but pulled his a**! Are you kidding me? Can we just [get a] round of applause for me?"