According to the wikipedia article Edward Misselden, he was born in 1608 and in 1623 became a deputy-governor of the Merchant Adventurers' Company at Delft. This means he got the job at the age of 15. Is it true? How did it happen? Was it normal at the time?
According to the Wikisource document that article is based on, Wikipedia accidentally left out an "fl." in the age span (i.e. it ought to be "MISSELDEN, EDWARD (fl. 1608-1654)"). "fl." (Latin Floruit) means "flourished", i.e. we know that Misselden was active in the period 1608 to 1654, but we don't know when he was born or died.
If that's true, then he probably wasn't born in 1608, and wouldn't have become a deputy governor in his teens (which, as you say, sounds pretty odd).
Edward Misselden was an economic philosopher. In 1622, he published this missive on free trade, which probably helped him get the deputy governor job.
Most other sources cite his lifespan as 1608-1654. That would make him 14 when he published the missive, and 15 when he became deputy governor. That would make him a child prodigy, an unlikely, but by no means impossible achievement.
In those days, few people of ANY age could read or write. Those that were wealthy enough to learn, and had enough talent to produce pieces like those above, were promoted to positions of authority as soon as their ability became obvious, because they basically had a monopoly of "brains" (intelligence plus education).
In a somewhat later (but similar) time, the Marquis de Lafayette of France was made a general (albeit in the AMERICAN army), while still a teenager. In those days, birth (and the resulting education) counted for more than age or "development."