Marriage is a secular matter, not a religious one. The government decides who can and cannot get married. Don't forget that it took a Supreme Court ruling to legalize same sex marriage. And you can't marry your cousin in the US. And then there's the question of avunculate marriage (banned in almost all states but acceptable, and encouraged, in much of the rest of the world)
Besides, absent that piece of paper called a marriage license (issued by the secular authorities) that church service is nothing more than a fancy ceremony with no legal import whatsoever. For that matter you can go down to City Hall and get married by a JP with no religious involvement whatsoever.
The age of consent in the US varies by state and ranges from 16 to 18 with 16 being the most common. BTW, there are exceptions - if the older party is in a "position of authority" (teacher, police officer, etc.) the age of consent is often higher (21?). And the age of consent for same sex activities is often higher.
And even then many states (but not all) have a "close in age" exemption (sometimes called a Romeo and Juliet law) that says that someone under the age of consent can have sex provided the older party is no more that a fixed number of years older (there is a minimum age below which someone can't give consent).
And that varies as you move around the world. For example, the age of consent in Canada is 16 [until relatively recently it was 14] but the close in age exemption says that a 14 or 15 year old can have sex provided the older party is less than 5 years older. Anyone younger that 14 can't give consent as a matter of law.