Brooke Shields Says 'Being Over 50 Is Not the End of the Line' but 'Beginning of a New Chapter' (Exclusive)

Brooke Shields isn't feeling down about aging up.

In an exclusive interview with PEOPLE on the set of GSK's Thrive@50+ campaign, the star, 59, opened up about the positives of growing older despite the stereotypical belief that aging is a bad thing.

"I've been aging for so long. I'm used to it by now," Shields says with a laugh ahead of the campaign launch. "I think that the messaging around aging is unfortunately predominantly negative. That is not the narrative that I am living — and we don't deserve to live that narrative."

Shields has undoubtedly lived an eventful life since stepping into the spotlight at age 12 as the star of Louis Malle's film Pretty Baby in 1978, a leading role that kickstarted her extensive career as a model and actress.

Simultaneous to her professional successes, Shields also became a mom of two daughters — Rowan, 21, and Grier, 18 — whom she shares with her husband Chris Henchy. The personal milestone changed her perspective on life.

"I think after you turn 50, so much opened up for me. I had raised my kids. I had a different place that I was putting myself in my life," she says. "I think I was never afraid of the next decade or the next year."

That's why joining forces with GSK's Thrive@50+ was a natural fit for Shields, who says there's "so much more" she's been "excited about" since turning 50. "This campaign really champions that ... and the message that being over 50 is not the end of the line. It's the beginning of a new chapter."

"It's so important to really understand the power that you have to take control of your health," Shields says of why she teamed up for the campaign, which celebrates those 50 and older and encourages them to learn about their risk for potential health challenges like shingles and spread awareness about vaccination.

When asked what the most annoying question she often gets about aging, Shields says "they always hit upon beauty for me." Though she says she's "not angry" when the question is posed, she acknowledges that it's "an adjustment."

"Yeah, I don't look like I did in my 20s... I'm talking like I had a lot of work done," jokes Shields, who's previously opened up about cosmetic procedures and the slippery slope of injectables. "But the questions that I'm asked are always usually in that beauty sector ... the vanity of it."

Shields accepts that she's "not going to be 20" ever again. "But I want to be the strongest and physically fit, and that I'm going to do all the things that I can do to make myself vibrant," she says.

Shields recently spoke with PEOPLE for her cover story in August alongside her two daughters for their first in-depth interview, where she opened up about her feelings as she watches her daughters grow older.

"I’m terrified to be an empty nester!" Shields admitted when thinking about their departure for college. Watching her daughters grow into confident young women, though, has left Shields in "awe."

"I’m going to really miss them," Shields adds. "I want to stay on the periphery but I’m also excited about what’s ahead.”