Also, mental dexterity and knowledge are probably inversely proportional to each other because once you learn more you draw on more to make decisions. So, if we could rid our minds of our knowledge we might improve our thinking speed.
Anyways, for people who feel down about their age, remember that nothing anybody does will last forever. Our judgment of significance is significantly colored by our finite sense of time. Just think of what seemed like a long time to you at 3 vs at 30. Thus, it is ok to primarily aim at happiness in this life instead of achievement, though aiming at achievement is also good if it makes you happy.
Regardless of thinking speed, the more you know, the better you think. You are able to analyze a given situation or problem more effectively because you can draw on past experience or knowledge and make a more informed decision. I would happily take an 'experienced' brain over a young on that is faster (if thats even true, which i dont really believe...).
Anyways, for people who feel down about their age, remember that nothing anybody does will last forever. Our judgment of significance is significantly colored by our finite sense of time. Just think of what seemed like a long time to you at 3 vs at 30. Thus, it is ok to primarily aim at happiness in this life instead of achievement, though aiming at achievement is also good if it makes you happy.