Throughout the novel I cannot remember a point in which Mark Twain tells his audience exactly how old Huck is. His independent nature and knowledge or survival methods lead the reader to think that Huck has had to be on his own for a while, which could mean that he is in his teenage years. His relationship with Widow Douglas and Miss Watson indicate that he needs to be cared for a lot, and that he is still of school age. That tells us that he could be much younger.
Twain seems to have purposely left the age of his protagonist ambiguous to reiterate his naivety and his malleability in his belief system and morals. It would be very difficult, if not impossible for an adult to be open enough to experience the transformation that Huck went through. Huck's impressionable mind had not yet formed its ideals about religion and morals, and his time in hiding with Jim allowed him to rethink everything that he had been taught in his brief life, and he was able to develop his own belief system.
The ambiguousness of both he and Tom's age forces readers to be less critical of their actions. As the reader learns view to Tom and Huck as young children, their immoral choices are less likely to be judged as harshly, such as when they kept Jim locked up even after they knew he was free. Their choice to play out Tom's elaborate plot is seen as a child's imagination running wild at the worst time possible, but during this time this plot and their previous moral code and belief system were not uncommon. Twain seems to show readers that the actions of the people during this time may be seen as a child-like ignorance, but they cannot be judged as such. If a child like Huck is capable of making his own decisions so should the adults of his generation.