In the mirror selfie, Simpson looks sassy as she poses with her hand on her hip to show off how well the distressed, 14-year-old jeans still fit.
"I have kept these throwback True Religion jeans in my closet for 14 yrs (I’m not exaggerating!)," she captioned the pic. "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."
Actress January Jones commented on the post, telling Simpson, "You look about 15 lady."
Earlier this month, Simpson geared up for her milestone birthday by posting a photo of herself in a cow-print bikini and cowboy hat.
"YEE-HAW to my final days in my 30’s," she wrote alongside the photo.