It wasn't unusual for children of this age to go to sea in the 18th century. The article points out how Royal Navy ships used kids as "powder monkeys", and the film "Master and Commander" has a very young Midshipman (Officer Candidate) as a major character. Children were often formally apprenticed to the sea-going life, and had many opportunities to serve, and we can speculate that some were not altogether wholesome.

We have to keep in mind that earlier times did not have the same sentimental attitudes towards children that we have, as attitudes to dangerous and arduous child labor in the "Satanic Mills" of the 19th century testify. Alas, there are parts of the world today where similar attitudes still reign.