If the partners are on different age category, which category should they register as?

E.g. it is common to have an adult category for 18-35 year olds, and senior for 35 to whatever.

If one partner is 36 and another is 34, then which category should they register? Do they have a choice?

It depends on the rules of the organization that is running the event.

In USA Dance (USABDA) events, *both* partners have to be turning 35 during the current year in order to dance in Senior I events. If one partner is 36 and the other partner will still be 34 at the very end of the current year, then they will have to dance in the "Adult" category, which is for 19-year-olds-and-up (but people can start dancing it in as young as 16).

Those are the USA Dance rules. Other organizations may be different, so take what I wrote as a sort of sample.

I just found this recursive rule in the NDCA rule book (p. 33 of http://www.ndca.org/information/rules/rules_pdfs/Rulebook_Master.pdf)

Here's an extract (irrelavent part edited out):

4. Amateur or Adult.
a. Adult A. Competitors who are 19 years of age or older.
b. Adult B. Competitors who are 35 years of age or older.

5. A couple's age classification eligibility is defined as the age of the older partner on the day of the competition for ... Adult “A” classifications, while it is defined as the age of the younger partner for Adult “B” ...

Now, if one partner is 18 and another is 36, it is not clear which category they belong to. If you put them in adult A, then the age of the older partner is the one that counts, so they really should be in adult B. But if they are adult B, then the age of the younger partner is the one that counts, so they should be in adult A.

What gives?

I meant to say one partner is 34 (adult A) and the other is 36 (adult B).