Addressing invitations

Standard etiquette is only use Mr for those over 18.

Yes to what Jessica said

I agree with everyone.

Old standard etiquette was Master between 12 and 18, then Mr., but that is rarely observed now. Most people address a young man as Mr. when he starts taking on some of the appearance and responsibilities of manhood. By 16 you may hold a job, can legally care for children younger than yourself, are allowed to travel interstate without any supervision, and in most states drive a car. So no later than 16 most people choose the address Mr. . No title has always meant either below 12, or below 14, depending on your area.

If they're on the same invite as their parents I would personally address it to the parents with titles and boys first names on the next line:

Mr. and Mrs. John Smith

Samuel & Joshua