Live long only if your brain is young

After reading Eric falkenstein's post "Why I Don't Want to Live Forever" I realized I need to put a few caveats before what I wrote about life extension, because my own enthusiasm for it rests on a few assumptions that I admittedly haven't bothered to spell out.

First off, note that Falkenstein's essay is titled "Why I Don't Want to Live Forever," not "Why I Think No One Should Want to Live Forever." And even though he argues beyond what's in the title--i.e. he argues that if people could live forever it would lead to negative externalities--it's good to see someone arguing against life extension without trying to tell people they don't really know what they want.

His main points are two. First, human mind deteriorates with age. I completely agree with him that life extension that would "freeze" people whose brains are, say, 96-years-old for a long time would be a disaster both for the person interested and for almost everyone around them. Life extension would only make sense if we could figure out the way to slow down the ageing of the brain so that it would match the new extended lifespan. His second point is this:

I see so many people develop a personality that after a while converges on some big idea, some routine in thought and behavior, it becomes not merely unhelpful to the world, but boring for the person and every thinking person around him.

It's true, this does happen all the time. But, not being able to observe people who live for hundreds of years, we don't know if this happens because of some physical constraints on the brain, or because it's an optimal response of the mind to the fact that life is short. That is, do personalities really "converge" or do people end up specializing in one single thing simply because they know they don't have time to reinvent themselves? Say you're a mathematician and that when you're 30, you get this great, career-making, Fields medal-worthy idea. For the next forty years, you harp on this idea, fleshing out its every possible detail and application. This happens a lot--but does it really happen because your personality has converged on the idea, or because you know your life is too short for you to become equally good at something else than that idea of yours? What if we could live for a thousand years and be able to say "okay, I've become so good at mathematics that it's become completely boring to me and everyone around me. I think I'll try photography now"--and then go and become a great photographer? Wouldn't such life be desirable?