As a 16 YO, I think I might agree with LissaR the most. Of course, I'm perfectly capable of engaging in adult conversation, its just that often they talk about things that I'm not exactly familiar with (tax problems?) or people that I haven't met, and so I'm left sitting there eating, feeling a bit awkward. Family get-togethers with my mum's side can be quite awkward, as I get lumped in with the adults, but unsure of whether or not they really want me there, or care for my attempts at contribution - but at the same time, I'd rather not to and play with the 10, 8, 6, & 4 YOs.
Conversation that you know teen will be able to discuss would be great, and offering the option of doing something else would take the pressure off of the pre/teen, so they didn't feel "trapped" into staying and being bored.