My understanding is that the guys have one “legal” wife, theoretically that one could be underage with parental approval.
BTW, what is the government doing in the deciding who can get married business in the first place?
Isn’t marriage a religious imperative?
I think you’ll have to go back a long way to get to a time when marriages weren’t registered with the civil authorities.
And while marriages at an early age have been legal forever with parental consent (you don’t have to go back many generations to see very young brides in most families) I don’t think you’ll be able to defend under-aged non-marriages of the kind we’re talking about.
Unofficial polygamy among adults is beyond the reach of civil law. But the catch is “adult”.