In an interview with Bustle published on Wednesday, "The Kardashians" star discussed her birthday milestone.

"When you're young, you think 40 is so old, and now I'm like, 'Wait -- I feel so good!' said Kardashian, who turned 40 in June.

"I am in the best shape of my life. I'm doing new career stuff. I've only been 40 for about six months, but it's the absolute best. You don't give a shit about some of the stuff you once did," she added.

She said she intentionally "tried to close a lot of chapters" at 39.

"I called it shedding -- shedding this decade of my 30s and this energy I wanted to leave behind. There are so many new things that I'm doing in my first year of 40 that I know for a fact my 40s and on are going to be fucking incredible," she said.

Turning 40 can be a daunting period. It's approaching midlife -- a "biological regularity comparable to a second puberty," Hannes Schwandt, an associate professor of human development and social policy at Northwestern University, previously told BI.

"When you're young, you are typically overestimating what you're getting in the future," he continued. But not everything in life turns out the way we expect them to, and this can lead to disappointment, Schwandt said.

Aimee Pearcy previously wrote for BI that she felt disappointed when she hit the birthday milestone. "I always thought I'd be a millionaire by the time I turned 40, but here I was, living in a van, broke," she wrote.

However, she eventually came to terms with her age. "I realized that age is just a number," she wrote.

"Now that I'm in my 40s, I feel like my life is just beginning."

A representative for Kardashian did not immediately respond to a request for comment from Business Insider sent outside regular business hours.