The fact that your grandson was a year younger than two of the boys in the group does not mean he couldn't have forced them to do what they did. Age has nothing to do with that. Your grandson could be bigger, stronger, more of a bully, brighter, or any one of a dozen other things that would make someone want to listen to him and do what he says.
Also, the police don't have to arrest everyone. It's not a defense to say, "The others were guilty too but nobody arrested them." It's like when everybody on the road is speeding, but you're the unlucky one who gets pulled over. You can still be charged for what you did, even if the others got away with it.
You may be right that he was arrested because Walmart recognized him. And if he goes to trial on this case, that's something that may be helpful. But it's not discrimination. In law, discrimination would be an unconstitutional arrest based on race, religion, gender, age, or disability.