I'm going to, respectfully of course, disagree on part of this. "Running away to sea" was the best way to change one's position in life, especially for those sons not born first. Women, too, disguised as men, also took advantage of this avenue to a lesser degree of course. Not having the data at my fingertips I would still venture that the majority of the sailors of the era were the sons of tinkers, tailors, farmers and shopkeepers.
The lesser sons of nobles also took this route, but as junior officers and midshipmen.
But at what age would this be done. I doubt that many of us would be welcomed on a ship without extensive knowledge of how to handle one. I say this because I believe that most of us are over the age of 21 with no sailing knowledge.