The age of the kid isn't given precisely. However, based on the story's context and the typical age for such adventures, one could estimate that he's roughly 9 to 10.
It's not specified exactly, but the kid is around 9 or 10 years old.
The kid's exact age isn't clear. But looking at the situations and activities he's involved in, it seems he's probably in the 9-10 age range.