Bindi Irwin, daughter of the late Steve Irwin, and her brother, Robert Irwin, are growing up fast, as evidenced by Bindi’s most recent Instagram post of the twosome.

Bindi Irwin Shows Off Darker Hair In New Photo

Bindi, 16, shared a photo of herself and her little brother, 11, on Saturday, writing, “This boy is my world.”

This boy is my world ❤️

A photo posted by Bindi Irwin (@bindisueirwin) on Jan 1, 2015 at 12:59am PST

The sweet photo quickly went viral, with fans noticing the striking resemblance between Robert and his father and Bindi’s darker hair color. Bindi’s hair has been growing darker over the years, from blonde to dirty blonde to more of a light brunette. Regardless of whether or not the teenager dyed her hair, as some outlets are speculating, Bindi is certainly growing up.

Bindi and Robert have remained in the spotlight ever since the sudden death of their father. Both have headlined animal television shows or events for the Australia Zoo. In fact, Bindi frequently updates fans on her Zoo adventures on Twitter or Instagram.

Bindi recently opened Bindi’s Island at the Australia Zoo in December, a “tropical paradise” where guests can see ring-tailed lemurs and other wildlife.

Super proud that Bindi’s Island is ready to be explored @AustraliaZoo! pic.twitter.com/3lRMtTa1IF

— Bindi Irwin (@BindiIrwin) January 5, 2015