Thomas Rhett's oldest daughter, Willa Gray, turned six years old on Monday (Nov. 1), and the country superstar can hardly believe how fast she's growing up.

"How in the world are you already 6?????" he writes on social media as part of a sweet birthday tribute.

"I love you more than life," the singer continues, "You are the best big sister any sister could ask for. You make me a better person just by smiling. Your smile could light up any room. I love getting to be your daddy. Happy birthday sweet baby."

Alongside his birthday post, Rhett shared a snapshot of Willa Gray that documents her infectious smile on full display. In the picture, she's wearing a white dress and gold crown and is surrounded by birthday balloons and other party decor:

Fans who have been following along with Rhett's sweet family story have known and loved Willa Gray since the singer and his wife, Lauren Akins, adopted her in 2017. Akins first met their daughter during a trip to Uganda, when she was 18 months old. The couple documented the adoption journey that ensued over the following year, and celebrated with their fans when they finally brought Willa home in May of 2017.

Just a few months later, Akins gave birth to the couple's second daughter, Ada James. Since then, Rhett and his family have welcomed a third baby girl, Lennon Love, and in May of 2021, they shared the exciting news that they were expecting another new addition — a fourth daughter. That baby is due in November.

Meet the Country Babies Born in 2021:

Welcome to the world, boys and girls! These babies are the newest members of the country music family, all born in 2021.