The ladies were all dolled up to get a head start on Jordyn's birthday celebration. She turns 21 Sunday but kicked things off Thursday with dinner at Petite in L.A. ... before heading off to Poppy.

Kylie turned 21 over a month ago and clearly couldn't contain herself at the thought of her BFF coming THIS close to legal drinking age. They capped off their dinner with a small chocolate cake and giant sparkling candle. Nope, no cereal ... with or without milk.

Bottle service was flowin' later that night -- at least for Kylie, who held up a giant poster with Jordyn's face on it.

They're obviously tight. Remember, Jordyn was front and center in Kylie's birth announcement and they're nearly inseparable -- except for that one time Jordyn flaunted the goods down in Mexico.

Happy turn-up!!