"Happy birthday to an icon living @noahcyrus," Miley wrote in the caption. "I love you baby sister turned big sister. The day you were born I never knew that one day the roles would be reversed and you would be the one taking care of me."
She credited Noah for helping her getting through the experience of losing her beloved pitbull mix Mary Jane, who died after a battle with cancer. On Jan. 7, Noah posted an Instagram Story paying tribute to the late pup.
"I couldn't have gotten here without you. You lead me to making the hard but right decisions," Miley added. "You'll have many more of those choices in your own life & I am here for you every step of the way."
She also included a word of warning to Noah.
"Happy 21st. Don't get near as drunk as I did," she wrote. "I practically missed my own party cause I spent the night over the [toilet.]"
Miley wasn't the only Cyrus to wish her little sister a happy birthday. Dad Billy Ray Cyrus also posted a sweet throwback video to Instagram of his daughter singing in honor of her big day.
"Happy Birthday @noahcyrus!" Billy Ray wrote in the caption. "Love you so much and so proud of you!"