We're moving on to a new era for the Kardashians. The catalyst for declaring a new epoch? Kourtney Kardashian's 40th birthday.

As the oldest of the Kardashian-Jenner siblings, Kourtney is the first to turn 40, entering a stage of mid-life that no other Kardashian sibling has entered before. For Kourtney, are the days of making mistakes in her twenties and learning lessons in her thirties, and here are the days of being a fully fledged 40-year-old grown-up.

That's not to say Kourtney hasn't been the Kardashian with the most responsible or grown-up behavior attached to her name in the past. She's always been a sophisticate, and the prospect of technically entering middle-age probably doesn't faze her in the slightest. In typical oldest child fashion, she's been the first of her siblings to reach plenty of life milestones, like having children or getting a college degree. In fact, her recent business venture is one of the few times she's not been the first of her family to do something, but with the launch of Poosh, Kourtney's lifestyle guide that serves as a sort of Goop for the Calabasas set, it seems like she is already set to live this decade to the fullest, balancing her first business venture with motherhood, and shutting down dating rumors on top of it all.

Her mother Kris Jenner already posted a birthday tribute to her eldest on Instagram, where she managed to shoe in a mom joke about how Kourtney will "always be my baby" even though she is the oldest of the bunch.

Kim Kardashian also took a break from acing her torts exam for law school and explaining how her bathroom sinks work to the Internet by sharing some birthday wishes for Kourtney via Twitter, where she included a handful of throwback photos of the two from over the years. "HBD @kourtneykardash Can’t believe the life we have lived! It’s such a dream to go through it all w having a sister like u by our side! You’re such an amazing mom &friend! Can’t wait to celebrate u tonite! I love you beyond what you could imagine," she wrote.

It probably won't be long before the rest of the siblings—Khloé, Kendall and Kylie Jenner, and even Rob—send their public birthday wishes to their biggest sister on social media. And if we can be certain about one thing with this family, it's that they know how to throw a good birthday party (and just good parties in general, for just about any occasion), so we expect her birthday bash to be at least as big as last year's, which doubled as an album listening party for her brother-in-law Kanye West's new music.

Related: Kourtney Kardashian Is Now Working With Stephanie Shepherd on Poosh