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.

What if I accepted the fact that I'll die of old age, and decided, after evaluating the arguments pro and against, that it's the best outcome for me?

Then thats how it is.

However, when someone reevaluates a long-held position, even one which was developed and maintained for manifestly irrational reasons, they are still extremely unlikely to change their mind. If someone claims to have evaluated such a position thoroughly, it is generally a good bet that they stopped thinking once they found a good enough justification of their previous belief, before there was any real risk of changing their mind. This is a central topic in the sequences.

Of course this is a bet, not a certainty. I don't mean to be harsh, but you should seriously consider the possibility that your argument is a rationalization. orthonormal's comment is probably more helpful to this end.

What about the preference reversal test? If you weren't constituted so as to age and die, how much would somebody have to give you in order for you to agree to self-modify to age and die?

If you genuinely (and not just as a devil's advocate) would place a very low price on switching in that scenario, then your values are indeed consistent and our criticism on this point might be misguided. If you find yourself reluctant in that hypothetical, though (and most people are), then you have an inconsistency, and you should consider the possibility that you're rationalizing in your thoughts about death.