So endeth this chronicle.
chronicle (記録) [名] a factual written account of important or historical events in the order of their occurrence
It being strictly a history of a boy, it must stop here;
strictly (厳密に) [副] in a strict manner
stop (終わる) [動] come to an end
the story could not go much further without becoming the history of a man.
go much further (これ以上先に進む) [動] proceed or progress beyond a certain point
When one writes a novel about grown people, he knows exactly where to stop—that is, with a marriage;
novel (小説) [名] a fictitious prose narrative of book length, typically representing character and action with some degree of realism
grown people (大人) [名] a person who is fully developed or mature
exactly (正確に) [副] in a precise and accurate manner
stop (終わらせる) [動] come to an end; cease to happen
marriage (結婚) [名] the legally or formally recognized union of two people as partners in a personal relationship (historically and in some jurisdictions specifically a union between a man and a woman)
but when he writes of juveniles, he must stop where he best can.
juvenile (少年少女) [名] a young person

 Most of the characters that perform in this book still live, and are prosperous and happy.
character (登場人物) [名] a person in a novel, play, or movie
perform (登場する) [動] act in a play or movie
prosperous (裕福な) [形] having a great deal of money, possessions, or other material goods
Some day it may seem worth while to take up the story of the younger ones again and see what sort of men and women they turned out to be;
turn out (なる) [動] to become or develop into something
therefore it will be wisest not to reveal any of that part of their lives at present.
wisest (賢明) [形] having or showing experience, knowledge, and good judgment