Princess Isabella is not the only royal celebrating a milestone this week.
On Tuesday the family stepped out to celebrate Queen Margrethe's 79th birthday but fans were most taken by how grown up the royal children looked.
Mary and Fred are also parents to eight-year-old twins, Prince Vincent and Princess Josephine.
WATCH this video to see Princess Isabella and her siblings celebrate their grandmother's birthday!
For her daughter's 11th birthday, Princess Mary shared a series of adorable photos showing young Isabella with her favourite horse, Lancet.
The official Instagram for the family shared the gorgeous pictures and revealed horse riding is Isabella's "favourite job".
This year, the Danish royal family shared a sweet snap Isabella smiling at the camera.
