First off, you do have a valid point that there are children who are more mature than adults.
But, saying someone is 10 in a disrespectful manner IS an insult because an insult is a disrespectful comment. If it's spoken with the intent to be an insult, it is an insult.
People may insult anyone any possible way they desire to. That doesn't mean it's right, but they have the right to. And generally I imagine a 10 year old wouldn't be as good at video games as someone older than they because they more than likely have more experience, knowledge, and coordination as a 10 year old.
I'm a teenager. I bet I'm better than the average 10 year old who plays LoL. But that's because my brain is more than likely more developed than the average 10 year old's brain and I'm more coordinated, less prone to childish fits, and smarter. Children have a lot to learn.
None of us are perfect, but there are valid reasons insults exist. As long as people are offended by something, being compared to said thing will be considered an insult.
People generally don't like being compared to something they aren't if there's an obvious tone of disrespect or loathing behind the words. The tone is just as important as the words.
Sorry, but not sorry.