Kylie even posted a picture of the two of them together via Instagram, wishing Jaden a happy 15th birthday Monday, reports femalefirst.co.uk.
The brand ambassador for US magazine Seventeen even attached a note with the picture.
It read: "Although we can't remember exactly when we met, I couldn't be more grateful that we did. You understand me like not many do & you always know the right time to make me laugh.
And no, you haven't failed once to catch me when I attempt to walk or open the door on my every entrance. Happy birthday to my best friend (sic)."
Even Kylie's mother Kris Jenner posted a picture of herself hugging Jaden, who is the son of actors Will Smith and Jada Pinkett Smith.
Her message to Jaden read: "Happy Birthday @officialjaden I love you Jaden!! Have the best day ever!! #bestkidever (sic)."
Kylie had earlier opened up about her special friendship with Jaden, saying they gel well because he understands what it's like to grow up in the limelight.
"He comes and supports me with everything that I do. It's refreshing. Some people don't understand and some people do.
And it's just great to have someone grow up in the same position as you and you can relate in that kind of way, so it's fun," Kylie, who is 15 too, had said.