It's the most common question I get from parents and seems pretty straightforward. But to get to the answer you have to think deeper, think beyond numbers. A more telling question is: what's his/her learning style?
Imagine you have a kid who can sit in one place, repeating the same action over and over for a long time without a single complaint.
Now imagine the same scenario, except that they're moving constantly, thinking out loud, struggling with repetition and taking twice as long.
Both children are on 2 different paths that lead to the same destination. One is just slightly bumpier and longer.
Keep in mind that if your kid is more of the second type, there's another question you have to ask yourself: how much time, effort and resources am I willing to provide to give them the support they need?
And if your kid is the first type, well you're a lucky parent aren't ya?
By the way, learning styles can always change and so can kids. How they learn today may not be how they learn tomorrow. And most of the time, if you can stay out of their way, they're smart enough to sort things out for themselves. Kids are a lot more intuitive and intelligent than most adults give them credit for. It's important to give them the space to freely express themselves.
So always remember, "age is just a number."