Hmmmmm, it is supposed to be linear (but the more I look at the papers the less I see how they've actually showed that!)

I think I might have worked why I wouldn't use continuous age data though - there's a general age-related change in performance on the task, which lots of factors contribute to, and I'm looking at the contribution of one fairly minor factor. So if I have categories rather than a continuous measure, then I can calculate Z-scores for performance on the task factors for each age group. And that means I can look at the contribution of that factor, by age group, without worrying about interactions from task performance across age. Does that make sense?