Naval conscription during 17th-18th century

[Janette Morgan]

I'm in the midst of research for a book of piratical nature, and I've come upon a bit of a snag concerning conscription practices of the British navy. My mother has provided an age range, and I am familiar with the practice of gang-pressing (or press-ganging; whichever it is), but I'm not sure of very much beyond those simple basics and I could use a bit of mathematical assistance.

My situation involves two boys, one of whom has no living parents and has been raised by his aunt, the other is the son of that aunt and somewhat older than his cousin. The nephew, being the more useless and generally irritating, was shipped to the Navy prompt at age twelve, and I am wondering whether it would be plausible for the son to remain at home for a year or two longer, to get in a bit more schooling and possibly make sure his mother will be all right on her own (she tends to suffer from athsmatic attacks). (All said and done, he would be between fourteen and sixteen by the time he left.)