In addition to the other replies, I'd like to add this. Most likely you're not very old right now, and dying "of old age" isn't an immediate prospect. So, saying "I want to die of old age in the future" is emotionally very different from saying "I want to die of old age (or something else) very soon".

Far vs. near mode comes into play, which means you'll tend say things about the future mostly for signalling purposes. Of course you're saying these things honestly, you really feel & believe them. But on the outside-view, there's a significant (though far from certain) probability that, when you really are old and dying is an immediate prospect, your preferences will change and you'd prefer a cure.