Restricting sub-categories

I'm building a site using Category with the following structure :

Categories :
* Age
- 18 and over
- under 18
* Themes
- sub-theme-1
- sub-theme-2
- sub-theme-3
- ...

The main node type is "Offer". Each Offer has one (and only one) Age category and has many Themes (possibly sub-sub-themes, actually).

Now, I want user to first select the Age category, and the proceed to the Themes. Certain Themes are available only if you are over 18, but most are shared. So, not wanting to duplicate a Theme tree for nothing, my questions are... :

- how do I get to display only the sub-themes for "over" and "under" 18?
- how do I get to display only the offers that are in both the right theme & age category?

Now, to make it even worse (and maybe I should keep this to myself) but the data is coming from a CSV file... I was planning to use importexportapi to deal with that, but it's not (yet) compatible with 5.x. I'll probably code that by myself...

Any help, comments, etc. would be most appreciated..