Kylie Jenner ''couldn't be more grateful'' for Jaden Smith.

The 'Keeping Up With The Kardashians' star posted a picture on Instagram and added a touching note to pay tribute to her ''best friend'' on his 15th birthday on Monday (08.07.13) , reigniting speculation they may be dating.

She wrote: ''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).''

Kylie's mother Kris also posted a picture of herself hugging Jaden - whose parents are Will Smith and Jada Pinkett Smith - and added her own message.

She wrote: ''Happy Birthday @officialjaden I love you Jaden!! Have the BEST day ever!! #bestkidever (sic).''

Kylie previously opened up about her special friendship with Jaden, claiming they get on so well because he understands what it's like to grow up in the limelight.

She said: ''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.''