Hilary Duff's little boy is growing up so fast!

The former Disney Channel star shared a sweet birthday message for her son Luca -- who turns five on Monday -- to Instagram over the weekend, ringing in the big milestone with lots of love.

WATCH: Hilary Duff Says Being Divorced 'Sucks,' Talks How ‘Terrible’ Dating Options Can Be

"Hey kid.... the past 5 years have been the BEST, hardest, most blessed years of my life," the 29-year-old actress wrote. "I love you so much. The world is yours my love ...I hope you take it❤️Happy Birthday Luca Cruz."

Duff also shared a cute photo of Luca rocking some serious attitude in adults sunglasses, jokingly captioning the pic, "Teenagers.."

WATCH: Hilary Duff: 'I Feel Like I Was Judged for Having a Baby Too Soon'

Happy birthday, Luca!

Meanwhile, Duff recently opened up about her divorce from Mike Comrie last year, after six years of marriage.

Watch the video below for more.