Kourtney Kardashian Shares Throwback Picture of Kylie Jenner for 28th Birthday

Kylie Jenner might be all grown up now, but her sisters remember when she was just a kid hanging out on the playground ... and, they've got pics to prove it!

The model and mogul turned 28 on Sunday ... and, Kourtney Kardashian posted several photos along with a loving message to her younger sis to celebrate.

The first pic will certainly grab attention ... 'cause it features Kourtney and Khloé Kardashian sitting on a bench alongside a very young Kylie.

The pic looks like it was taken about two decades ago -- Kylie's not even a teenager yet -- and the trio are enjoying an assortment of breakfast foods ... bagels, fruit, etc.

Kylie's got a huge grin on her face ... but, we gotta admit, the Kardashian girls look a lot less excited about their outing.

Other photos in the set include far more recent glam shots with Kylie and Kourtney hanging on to one another. KK captioned the post ... "Tinker Bell! May you shine forever my little goose sister. ✨❤️" -- seemingly a sweet inside reference.

The reality star's mom Kris Jenner also shared an assortment of throwback pics for her youngest daughter's birthday and gushed about how "beautiful inside and out" she is ... also calling her "the best daughter, mommy, sister, auntie, and friend."

Ky's not quite thirty and flirty yet ... but, she's definitely thriving -- as evidenced by all the hot shots we've put together for your viewing enjoyment!