I would interpret the statement like this:
The first part is almost Marxian. He is saying that the middle class (though necessary for a democracy), longs to rise to the status of the wealthy, looking up to them, fawning over them, while keeping the poor where they are. And so, the middle class tends to maintain the status quo, which means that society cannot advance, in fact remains conservative, if not reactionary. Terrible things may be done in the society's name, so long as the middle class is happy and satisfied.
In other words, it explains why America, once the most admired of nations, full of pride and idealism, is now blustering and fearful. It explains why we are in Iraq, why we are so despised.
The second part of the statement may be related to the first, in that we tend in youth to take chances, explore, be playful, and learn, but by middle age we have often become satisfied, safe, greedy, and careful. We decide, on a basis of our experience, what can be done and what cannot. We begin to value property and possessions over life. In other words, we begin to die. And in old age, we are either at peace with our accomplishments or regretful of opportunities missed. We are spent, and use whatever power we have gained or kept to hang on to our possessions until we die. We tend to become the walking dead.
My guess is that Welles, more than most men, would have given anything to have remained 26 his entire life. He had so much to offer, and he was getting so much done.