Despite an official minimum age for midshipmen, captains would at least occasionally stretch a point and take on a younger boy, most often as a favor to a particular friend or influential person. Getting in more sea time early on meant being able to pass for lieutenant earlier, which at least offered the possibility of becoming a post captain more quickly. In many cases, as O’Brian mentions repeatedly throughout the books, boys would be carried on a ship’s books even when they were not in fact on board, “officially” accumulating sea time while still at school or at home; while very much against regulations (signing a false muster roll was grounds for dismissal from the service), it was nevertheless quite a common practice.
Jack frequently has occasion in the books to declare his preference for having no “squeakers”, no “first voyagers” on board, particularly when in command of the Surprise, she having little room for anyone not able to take full part in sailing and fighting the ship. Blakeney was pressed on him by persons he could not afford to offend. And on at least a couple of occasions, O’Brian seems to call attention to the patent absurdity of a boy of 10 or 12 giving orders to men four times his age and twice or three times his size, though I don’t recall anywhere that he has Jack reflect on this directly.
[spoiler]
As for the ending, I’d say Jack’s main concern is preventing the French captain and her former crew from retaking the vessel; this would inconvenience Jack and the rest of the Surprises in several ways – obviously Jack doesn’t want Pullings to be killed or captured, nor any of the rest of the prize crew, but something that’s not obvious from the film is the emphasis in the Royal Navy of the time on prize money. Once Pullings gets the Acheron to a convenient port, she’ll be condemned and sold, with the crew of the Surprise sharing in the proceeds (3/8 to the captain; 1/8 shared equally among the commissioned officers including the marine captain; 1/8 shared equally among the warrant officers and marine officers; 1/8 shared equally among the midshipmen, carpenter’s mates, gunner’s mates, boatswain’s mates, sailmakers, yeomen of the sheets, coxswains, marine sergeants, etc.; and the remaining fourth to be shared equally among the remainder of the crew, including seamen and marines). Even for a military vessel like the Acheron without any costly cargo on board, the proceeds would probably put more than a year’s pay into pockets of every man aboard the Surprise. And you’ll recall that the Acheron has captured several British whalers, and some of their cargo has presumably been moved into her.
If the Acheron’s captain retakes her, they stand to lose both friends and fortune. The Surprise could ill afford to send over a large prize crew to take the Acheron into a South American port (having lost many men in battle and to accident and illness), and given the custom of the times the French captain (impersonating her surgeon) would likely have been given liberty of the upper decks upon giving his parole not to attempt escape or retaking of the ship. Having deceived Jack by masquerading as a surgeon, the French captain would probably not scruple to violate his parole either, so freeing the far more numerous French prisoners below and retaking the ship would be a distinct possibility.[/spoiler]
On the issue of flintlocks vs. match locks on the guns, Jack seems genuinely conflicted. On the one hand, he is a sea officer, and as O’Brian points out repeatedly throughout the books, seamen and their officers “like what they’re used to”. Match locks were the standard in Jack’s youth, he is used to them, and he has the standard naval abhorrence of anything smacking of innovation. On the other hand, he seems to recognize the advantages of flintlocks for rapid gunnery, and the ability to fire quickly and accurately is something he prizes highly in a ship’s company. His concesssion to his irrational mistrust of the newfangled flintlocks is to keep slow-match smoldering in tubs close at hand in case the flintlock misfires.