While his exact age isn't explicitly stated, it's clear from the text that he is young enough to be considered a child and old enough to attend school. He is also described as being "small for his age", further suggesting he is around 10.
However, the age of characters can vary across different adaptations. For example, in the 2005 movie "Charlie and the Chocolate Factory" directed by Tim Burton, Charlie is played by Freddie Highmore who was 12 at the time.