I have to ask, what is your age that you think you only have 20 years of brain power left? For sure, there are ages for which this is true, but given the age distribution of HN readers, seeing this post makes me curious.

Perhaps the plasticity of our brains is reduced, but that alone doesn't mean you have no brain power.

If you think of learning as "leveling up", you may stop increasing in level at some point, but you are still very powerful and productive.

I do agree with the sentiment of your last sentence however. We should endeavor to make our "leveling up" years count, and who knows, maybe you level up for the rest of your years.