The reality TV mogul shared several photos in honor of her longtime pal -- who turned 27 Sunday ... and, on her IG story she went with a pic from their childhood -- showing them goofing around in pajamas.

Look at the pic ... young Kylie and Stassie had a fun time back in the day -- hitting poses and cracking up while the camera caught the candid moment.

While Kylie and Stassie used to do PJs and sleepovers, they've obviously embraced night life these days -- including last night when the two partied hard ... crushing shots and clutching each other tight.

Kylie captioned her post, "for liiiiife. ♾️♾️" ... so clearly their friendship's as strong as ever.

Stassie posted in honor of herself too ... though she's also gettin' that birthday bag it seems -- sharing a post sponsored by Grey Goose.

If you don't know ... the two pals are super tight -- Kylie says Stassie's her oldest friend -- and, they've both gotten into modeling and influencing over the years. SK boasts more than 10 million followers on IG.