I think what Erik is talking about is different than what you're talking about. Erik's hypothetical 19-year-old ambition machine wants to be CEO of a company and cure cancer and invent a way to feed all hungry nations and then become president of the US by the time he's 40. You're talking about personal growth in one's chosen professional field. I think there's an important distinction here.

I also think that Erik kind of hit the nail on the head in his summation: at a certain point, you become comfortable wih life as you know it. Instead of thinking that you have to solve all the world's problems to be happy, you realize that happiness can come from much simpler sources, and that one's purpose in life is finely fulfilled by that happiness. Furthermore, as Erik said, one acquires responsibility that prohibits one from a realization of their 19 year-old selfs' goals, and that makes you reevaluate your life trajectory.

I think the issue up for debate is who has got the right attitude on life- the jaded mid-20s worker or 19 year-old ambition machine? Have we sacrifced something important as regards life goals in order to find the stability and comfort that we currently enjoy? Is it better to live a generally shitty life with grand goals and aspiration (and a small, but possible shot at acheveing those goals) or is it better to resign one's self to a life of quiet work with simple, comfortable, and ultimately happy-making pleasures?