Ellen Pompeo reveals the one reason she'll never leave Grey’s Anatomy.

This fall, Grey’s Anatomy will begin it’s 13th season.

Of the original cast of series regulars, only four remain on the show. Ellen Pompeo has stayed around the entire time.

But why? Katherine Heigl, Sandra Oh, Patrick Dempsey and many others have left. Why has Pompeo stayed? The answer? Aging.

“My decision to stay on [Grey’s Anatomy] was based solely on age,” the 46-year-old actress said in an interview with People. “At 33, I was wise enough to know my clock was already ticking in Hollywood.”

Compared to many actresses, Pomepo got what she calls a “super late start” in the industry.

She didn’t land her first role until she was in her mid-20s. That being said, she admits that if she had booked the role of Meredith Grey at an earlier age, things both for her and the series would probably have gone differently.

“I probably would have done my time, then gone out to search for other things,” she explained.

Pompeo says it was contract negotiations that pushed her decision to stay on the show so long.

With every renegotiation she became more and more “aware of how challenging it would be to find other roles in my late 30s and early 40s.” So she stayed.

But don’t think woe is her, she is now one of the highest paid actresses on TV.

“I thought, ‘Why would I leave something that is super successful and pays me great to search the landscape?’” she said. “I decided that I would stay on ‘Grey’s’ and be grateful and try to ride it out for as long as I could. And I am very happy that I made the decision to stay. It’s only gotten better with time.”

Though she does admit it's getting harder to watch herself. “Watching myself age on TV is miserable. To be honest, it’s the toughest part of my job,” she explains, calling older episodes, "intense."

“But as uncomfortable as watching myself age is, I don’t think focusing on physical beauty is necessarily the best thing for your mind," says Pompeo. "It’s a natural thing that we all do, but I don’t think it’s the healthiest thing. The older you get, the more you realize life isn’t about your looks. Everyone ages, and it’s okay.”

Pompeo now wants to bring that sense of fearlessness and humor around aging to other women. She has teamed up with beauty brand Philosophy in a new tongue-in-cheek beauty campaign.

"Welcome to middle age," the ad's voiceover says. "From here on it’s a steady decline into old age and cultural insignificance."

Pompeo then acts out some of the expectations society places on older women. We see her winding down her career, exercising less, wearing more modest clothing and of course, letting herself whither away.

We don't see that happening to Pompeo anytime soon.

“I was flattered to join the campaign because it speaks to stereotypes I feel strongly about," Pompeo told People. "As a woman you’re not over or on the decline after having children or after 40. It all just gets better!”