Not a baby anymore! As one of the stars of Teen Mom 2, Kailyn Lowry and her son Isaac have grown up before the eyes of MTV audiences. 

"Happy 5th birthday to an amazing little boy who made me MOMMY. No amount of words or Instagram posts can justify how much I love you! ***insert 'my wish' by the rascal flatts lyrics here***," Lowry captioned a photo of her little one's Ninja Turtles-themed 5th party. 

Lowry, 22, had Isaac with her ex Jo Rivera, but has since moved on to Javi Marroquin, whom she married in September 2012. Lowry and Marroquin welcomed son Lincoln in November 2013. 

To honor his step-son's big milestone, Marroquin posted a photo on Instagram, writing, "I wasn't sure if I was ready, wasn't sure what I was getting into, wasn't sure if I was able to provide, wasn't sure about a lot of things. Do I jump and hope I don't fall? You were one of the best decisions I've ever made. I remember when I first met you, Isaac. Barely talking and a head full of curls. 4 years later you and your brother continue to teach me what a dad really is. Happy 5th birthday Papi JR. I love you."