hm_jollies | Another query

I've come across references to boy Marines, but again I'm not sure of where they fall in with the duties &c of a detachment. These aren't drummers, but regular Marines, if younger than most.

Their duties? Powder monkey in battle, possibly nippers when weighing anchor (check this) but otherwise?

Were they still training? Learning musket drill?

Pay?

At what age did they stop being mustered as 'boy Marines' and start mustering as adult Marines?