Most English speakers know that the usual way to make a noun plural is to add -s to the singular: boy/boys, knight/knights, house/houses. They are also aware that the plural of few nouns, like child and ox, is formed with the quaint ending -en: children, oxen.
The modern convention of forming a noun plural by adding -s was not a foregone conclusion. Old English formed noun plurals with five or six endings, depending upon which declension the noun belonged to and how the noun was used in the sentence.
By the Early Middle English period (1100-1300), most of the OE inflectional endings had dropped away, but the plural ending -en was still in the running with -s and -es:
for a time, at least in southern England, it would have been difficult to predict that the s would become the almost universal sign of the plural that it has become. Until the 13th century in the south the -en plural enjoyed great favor, being often added to nouns which had not belonged to the weak declension in Old English. –Alfred Baugh, A History of the English Language p. 191.
We’ve seen how the apostrophe was added as the mark of the singular possessive because of a mistaken notion that a letter was missing before the -s.
Once the ’s became established as the possessive ending of a singular noun, it was inevitable that the apostrophe would also be used in the formation of plural possessives.
Although many writers misuse the apostrophe in the plural possessive, the rule is quite simple, in both American and British usage:
If the plural ends with -s, add an apostrophe: the boys’ kites, the knights’ chargers, General Motors’ mission statement.
If the plural doesn’t end with -s, add ’s: the children’s teacher, the oxen’s yoke.