Amber Portwood and Gary Shirley's daughter Leah is growing up quickly, and Teen Mom fans can't get over it.

When Amber and Gary made their Teen Mom debut in 2009, they were just 18 years old.

The former couple welcomed their daughter, Leah Leann Shirley, in November 2008 before breaking up in 2011.

Gary currently has primary physical custody of Leah, who lives with him, his wife, Kristina Shirley, and their younger daughter, Emilee Grace.

Leah and Emilee were heading back to start another school year earlier this week.

At 15 years old, Leah entered high school as a sophomore this year, and Gary uploaded a photo of the teenager with her little sister on their first day of school.

Leah looked all grown up in a new picture, wearing her long hair in loose waves. She looked stylish, sporting a black bodysuit, navy blazer, and distressed jeans, and wore clear-rimmed glasses for her first day of school.

Leah's little sister also wore her hair down in loose waves and opted for a light blue dress for her first day of fourth grade.

Leah and Emilee posed on the front porch of their Indiana home with their arms around each other in the adorable sibling snap.

Gary set the photo to the song Isn't She Lovely by Stevie Wonder, and in the caption, gushed over his daughters.

"2 of the the most beautiful girls, love you both very much!" Gary wrote. "@itsleahtime12 @kristina_shirley3 #anyone feel free to share your first day of school pics:)"

Not only did Gary gush over Leah in the caption, but so did Teen Mom viewers, who headed to the comments section to flood the post with compliments.

"Omg Leah got so big now and gorgeous," wrote @pretty_2_society.

Another Teen Mom fan expressed how "amazing" it's been watching Leah grow up.

"I remember watching Leah go to kindergarten on teen mom! time really flies!!" they added to their comment.

Another Teen Mom viewer reminisced about watching Leah as a baby "walking around in her mom's high heels" on the show.

"How can she be in high school?" they pondered.

Others added how Leah is growing up "so fast," is her "moms twin," and how "gorgeous" and "beautiful" the teenager looked in her back-to-school photo.

Teen Mom viewers know that Amber and Leah's mother-daughter relationship has been strained over the years.

Amber -- who has been inactive on Instagram since December 2023 -- opted not to post any photos from Leah's first day of school.

It's unclear if she was present for the photo op or if she chose to keep the moment private and off social media.

In the meantime, Leah's Instagram activity indicates that she's living her best life, with or without Amber in the picture.

Last month, Leah enjoyed a trip to Pink Lily with Kristina and Emilee.

In her caption, Leah wrote, "Shopping day at the @pinklily Warehouse sale with @kristina_shirley3 and my sisters!!😁."

In June 2024, Leah posted a carousel of photos from her family's trip to Clearwater Beach with her family.

Leah shared photos from the plane ride, in front of the palm trees, by the pool, and showed off her fashionable side in a green sundress.

"Spent the week at Clearwater Beach with my family!!☀️🌊," the 15-year-old captioned the memories.