On May 7, Millie penned a sweet message to her leading man, sharing it on Instagram alongside a series of photos with Jake. "this sums us up. In bliss, exploring together," she captioned the pictures, including a shot of them at the Enola Holmes 2 premiere in October. "I am eternally holding your hand. happy birthday fiancé. I love you."
The 19-year-old's celebratory post received a flood of well-wishes from fans and fellow celebs, including Paris Hilton who commented, "So happy for you."
Millie's Stranger Things co-star Matthew Modine also wrote, "Life is a beautiful long walk. There is a beautiful African saying : if you want to go fast, go alone. If you want to go far, go together. I love you both and enjoy seeing you walk together."
And although the couple has received a ton of congratulatory messages on their engagement, there's also been some skepticism, given their young ages.
In fact, Jake's dad even weighed in on the conversation during a conversation with Andy Cohen on May 2. "I don't know if age matters," Jon said on SiriusXM's Andy Cohen Live. "If you find the right partner and you grow together, I think that would be my advice really: growing together is wise. Growing together."
The "Livin' on a Prayer" singer—who also shares kids Stephanie, 29, and sons Jesse, 28, and Romeo, 19, with his high school sweetheart Dorothea Hurley—added, "I think that all of my kids have found the people that they think they can grow together with and we like 'em all."
As for how Jon feels about his son's leading lady? "Millie is wonderful," he noted. "Her whole family is great, really great. Jake is very, very happy."
To see a complete timeline of Millie and Jake's relationship, keep reading!