When I was, I think, 13 I pranked a disabled kid
Why did you want to prank the disabled?
Just for the burst of laughter.
You had no right to bully the disabled.  Even though you were just a child, you should be ashamed of what you did. That child might have suffered immensely because of you and may have been mentally harmed just because you wanted to laugh.
The kid was fat.
The fact that he was fat is completely irrelevant, and I really hope you don't think that that excuses what you did as a teenager.  People with disabilities, either mental or physical, have to suffer mistreatment every day of their lives, and you have no right to make it worse for them.