Fans of the Wales were enthusiastic about the throwback photos.
"More like, Kate enjoying her youthfulness and now she's a grown-up, so she has obviously matured," one user wrote.
"They were having the best days of their lives. just like any of us here during college," another added.
"We hadn't expected Kate and her family to turn up, and myself and a number of photographers and journalists were all standing on a press stand together," Rebecca English said in William & Kate: Too Good To Be True when discussing Kate attending William's 2006 passing out parade.
"Suddenly, out of the corner of her eye, we saw Kate and her mother and father being ushered down to the front of the seating," English stated. "I remember turning to the person next to me and saying, 'That's it, it's a done deal. This woman is going to be our future queen.'"
Although the Wales didn't wed until 2011, the 2006 gathering led many royal journalists to predict the pair would walk down the aisle.
"Suddenly, the engagement whispers were starting," she continued. "There was intense pressure on William and Kate, and, short of printing the tea towels, everyone had these two walking down the aisle imminently."
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"The low-slung jeans, denim skirts, and spaghetti straps of her early 20s were swapped out for smart blazers, quality suits, and refined gowns, with a more polished, Palace-ready look emerging," she explained.
Tom Quinn shared a similar sentiment in the unscripted project.
“I think that did come about because William and Kate were coming towards the point where they would announce their engagement. It was Kate starting to behave in ways which would be seen more appropriate for a future queen," he added.
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Fashion expert Miranda Holder analyzed how Kate's attire shifted when she entered her 40s.
"I’m pledging my allegiance to these charities and causes. … I’m your future queen, and I’m doing the best I can,'" she added. "It’s sending out a message of business rather than fashion. … [She’s] trying to take away from the world’s — including my [own] — fixation on [her] fashion."