It is what soldiers of the Pewdiepie army are called. They were previously called as Bros, Sisters, Sister Sqaud, Squad and are now 9 year olds. True 9 year olds are those who did their part, such as Mr.Beast. Do your part, and subscribe to pewdiepie
The 9 year olds pledge their loyalty to the Lord Pewdiepie.

The 9 year olds suffered casualties in the T Series war

My friend Imag Ayman enrolled in elementary school to become a 9 year Old
by Brief Velkoz December 5, 2018
If it's a 9-year-old, it's subscribed to PewDiePie.
I've got 9-year-olds of worlds so hold your defecation.
by SociopathicDuck January 1, 2019
What Pewdiepie calls his fans.

History:
PewDiePie uploaded a reaction video, Alinity made a response video (I can’t call it an apology video just because), and that video was reviewed by PewDiePie. Alinity mentions PewDiePie having 9-Year Olds as his fanbase.
Alex: Hey, Cam. I've you calling your friends' 9-year old.
Cam: yeah.
Alex: What's that supposed to mean?
Cam: It means that you aren't one, even tho you should be. subscribe to pewdiepie
by Gllllow January 14, 2019
Fans/subscribers of Pewdiepie
Anyone who watches Pewdiepie, regardless of their age, is referred to as a 9-year-old.
"I love my 9-year-olds!" -Pewdiepie
"If you're subscribed, you're an actual 9-year-old."
by RealGodsSub2Pewdpiepie February 8, 2019