When it comes to the Adult B (also called Senior I, also called 35-and-over) and the Adult C (also called Senior II, also called 50-and-over) categories, it is the age of the YOUNGER couple that determines the eligibility.
A dancer who is 16 may dance up into the Adult A events. Therefore, when you have a couple where one is 18 and the other is 36, they may only dance in Adult A.