• Jessica Simpson kicked off her 40th birthday with a pair of 14-year-old jeans.
  • The singer shared an Instagram photo of herself sporting the “throwback True Religion jeans” on Thursday night.
  • “I figured that since I’m in the final hours of my 30’s I’d give them another try, and hello 40, so nice to meet you,” Simpson wrote in the caption.
  • Visit Insider’s homepage for more stories.

Jessica Simpson is kicking off her milestone birthday with one very special throwback.

Simpson, who turned 40 on Friday, started celebrating a day early with the help of some 14-year-old jeans that had been collecting dust in her closet.

The mother-of-three shared an Instagram photo of herself sporting the “throwback True Religion jeans” with a purple tie-dye sweatshirt on Thursday night.

http://instagr.am/p/CCcYC-Tj6nh

"I figured that since I'm in the final hours of my 30's I'd give them another try, and hello 40, so nice to meet you," Simpson wrote in the caption.

Simpson's post has already racked up more than 290,000 likes at the time of writing, along with plenty of praise from fans - including January Jones.

"You look about 15 lady," she wrote.

"I remember those jeans!" added celebrity hairstylist Ken Paves, who has worked with Simpson in the past.

While some people might dread a new decade, Simpson has been looking forward to the big 4-0.

Simpson shared an Instagram photo of herself rocking a bikini and cowboy hat on June 26, writing in the caption: "YEE-HAW to my final days in my 30's."

It has been a whirlwind year for Simpson, who kicked off 2020 by releasing her candid memoir, "Open Book."

Simpson didn't hold back as she delved into her past, discussing her relationships with John Mayer and ex-husband Nick Lachey, as well as the pressure she felt from Hollywood when it came to her weight.

http://instagr.am/p/CBzNWiTDaOq

The singer said she took diet pills for 20 years after music mogul Tommy Mottola told her to "lose 15 pounds" when she was 17 years old.

"I thought it was about my voice. I didn't know that it was going to end up being about how I looked in a dress," Simpson told People in February. "It's heartbreaking and, I mean, I punished myself for it. I took diet pills. I heard it and I couldn't not hear it in the back of my mind every time I was on stage, every time I walked out the door."

Simpson has also opened up about her recent weight loss, revealing that she felt "like myself again" after giving birth to her third child Birdie Mae.

Harley Pasternak, Simpson's longtime trainer, said she dropped 100 pounds by walking 14,000 steps a day and prioritizing sleep.

And while Simpson has tried Weight Watchers and smoothie diets in the past, Pasternak said he didn't want her to feel restricted.

http://instagr.am/p/CAstLVJjEDP

"It had to be more than just getting back from her baby weight, but how do I keep whatever I am doing now forever," Pasternak told E! News. "That's why we are not a big fan of doing any extreme diet or radical forms of exercise."