By states laws all married couples must confirm to the new state's laws. If the age of married in state X is higher than state Y state X nullifies the marriage. If the states have the same age of marriage with parental consent the couple must provide documentation of parental consent and the court order validating the consent of the couple and parents.
Yes as immigrants are covered under the executive branch and foreign affairs department which are bound by UN regulation which state marriiage under 18 is against international law, the UNDHR, Convention on Consent to Marriage, and Minimum Age for Marriage and Registration of Marriages. Each of which states tradition, culture and religious customs which are against these resolutions are to be removed from civilization(s).
Yes there are as per the cited resolutions and conventions above. Nations just selectively follow the UN when each nation pleases. OIC nations never signed the UNDHR a key part of the UN charter but each is still a member. This only shows the UN is toothless without the US calling the shots.
Yes those marriages must conform to state laws if they are citizens
Nope it is 16 in Canada as per "Zero Tolerance for
Barbaric Cultural Practices Act" More so Canada has repeatedly denied immigrants and spouse-sponsored immigration which violate Canada's marriage laws.
CanLII - Zero Tolerance for Barbaric Cultural Practices Act, SC 2015, c 29