I don't think he believed he would live to 150, but saying that expressed his feeling of something that was beyond his reach but which animated every moment of his life and his art.

What I take from the quote is that existence shouldn't be a bell curve, with half or more of your life as an inevitable decline, but that your youth can be a foundation upon which to build knowledge and wisdom. You should end up smarter after many years, shouldn't you?

To be able to look back and smile at one's bravado, knowing that you can do so much more now, with less effort and rush, and that if you continue striving, you'll do even more in the future. To deeply and truly understand your craft.

That's what I'd want, a real life worth living, not to be the tech equivalent of a child star.

Yes, except for the pesky problem where your mind weakens and often falls apart entirely after not that many decades.

If you find that you are declining mentally after 30 or 40, you are either doing something wrong or are genetically extremely unlucky.

Exercise, a good diet, proper sleep, combined with plenty of intellectual and social interactions seem to be enough to keep you from measurably losing any mental faculties well into your 70s or 80s (and even 90+ if genetically lucky).

Also technically incorrect on the mental decline as well - our brains do start to irrevocably decline at around age 30. Our myelin sheaths fully develop in the early 20s, and from there, we have a few golden years until it's all downhill :D http://www.sciencedaily.com/releases/2009/03/090320092111.ht...