The birthday girl and her nearest and dearest celebrated on Sunday with Kendall blowing out the candles on a delicious-looking white-frosted cake at a masquerade bash, during which they donned bejeweled masks.
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This was after proceedings go off to a start last Tuesday, when the family rented out LA theme park Six Flags Magic Mountain.
Looking on this Sunday were sisters Kim Kardashian, Khloe Kardashian and Kylie Jenner, mum Kris Jenner as well as Kendall's friends Spencer Johnson, Nicolette Sage and Skye Savoy.
Leading the felicitations on Twitter was Kim, who wrote: "Happy 18th Birthday to the sweetest girl I know @kendalljenner I hope your adult years are filled with happiness & so much."
The message accompanied pictures of Kendall as a little girl with ribbons in her hair and another of her as a young teen, glammed up for what looked to be a modelling shoot.
Khloé chipped in with a photo captioned "birthday girl", of herself and her sister in masks.
Earlier, the emotional sister had shared a collection of pictures of the two together, writing: "Happy birthday Kendall!!!!! 18 years ago my life was changed forever because of you!
"The joy and peace you bring to so many lives are just a few of the rare gifts your spirit possess. I couldn't be prouder of the woman you are becoming. Happy birthday to my best friend, my roommate, my puppy sister, my heart. Sisters by blood, best friends by choice."