I don't think that makes any difference. He was 20 therefore he didn't know it was bad?? It wasn't just racist things Robinson said either
I think it's quite a lot more common for 18/19/20 year olds to make serious errors of judgement about what is acceptable than it is for someone in their late 20s, yep.
I would also say the most pertinent difference is that Robinson posted tweets that he presumably thought were poor taste but private and not directly addressed to anyone. Ballance was actively directing racial slurs to Rafiq's face.