He's a 30-something salaryman who knows everything. Or something like that.
In a modern and industrialized country with public education, it is possible to have a vast store of knowledge before reaching the age of 18. Or even 16. Math and science, literature and arts, history and political science, economics and industry; these are all part of a public education to some degree. He knows the nature of the stars and planets, and has a passing idea of the laws that govern them. He may have learned about the formula for making gunpowder, and the importance of rifling a gun barrel. He would know how to keep sailors from getting scurvy on long sea voyages, and maybe have some hints about the longitude problem in ocean navigation. Such knowledge, with some development by experts, could transform any kingdom into a superpower.
It is strange to think that such powerful knowledge is given to every child before they become adults.