Is it a fact about all preferences that they hold from birth to death? What about brain plasticity?

It's a term we're defining because it's useful, and we can define it in a way that it holds from birth forever afterward. Tim had the short-term preference dated around age 3 months to suck mommy's breast, and Tim apparently has a preference to get clarity about what these guys mean when they talk about morality dated around age 44 years. Brain plasticity is an implementation detail. We prefer simpler descriptions of a person's preferences, and preferences that don't change over time tend to be simpler, but if that's contradicted by observation you settle for different preferences at different times.

I suppose I should have said "If a preference changes as a consequence of reasoning or reflection, it wasn't a preference". If the context of the statement is lost, that distinction matters.

That's rather startling.

I agree! Consider, for instance, taste in particular foods. I'd say that enjoying, for example, coffee, indicates a preference. But such tastes can change, or even be actively cultivated (in which case you're hemi-directly altering your preferences).

Of course, if you like coffee, you drink coffee to experience drinking coffee, which you do because it's pleasurable - but I think the proper level of unpacking is "experience drinking coffee", not "experience pleasurable sensations", because the experience being pleasurable is what makes it a preference in this case. That's how it seems to me, at least. Am I missing something?