Carrie Underwood's firstborn son, Isaiah, turned 5 on Thursday (Feb. 27) and the singer — who has often documented her eldest child's sweet and caring personality on social media — took time out to reflect on his latest milestone.

"He is five today. Where did these last five years go? It’s so bittersweet watching your children grow up," Underwood wrote on Instagram, adding a precious photo of herself holding Isaiah as a newborn. "You want them to stay little and chubby and cuddly forever but it’s also so much fun seeing them turn into little people!

"Isaiah, you are funny and smart and such a caring little boy! You already have so much love for God and for all those around you. The day I became your mother was the day my life changed forever. You are five today but you will always be my baby boy...no matter how old you get. Thank you for being you."

Underwood keeps fans amused regularly with Isaiah's escapades, which include a crush on Maddie & Tae's Maddie Marlow, and funny preschool assignments in which he discusses his (apparently 70-year-old) mother's love of doing laundry.

Isaiah is big brother to Underwood and husband Mike Fisher's younger son, Jacob, who just turned 1 in January, and was described by Mom as being "smiley, crazy, smart, soooo fast and into everything!" Jacob was the happy result after the couple suffered "multiple" miscarriages before finally experiencing a healthy pregnancy.

Underwood has gotten the family acclimated to being on the road last year for her Cry Pretty 360 Tour, but she has not announced a 2020 trek yet.

Carrie Underwood and Mike Fisher's Sweetest Pictures Together: