In the first chapter of Walden; or Life in the Woods, Thoreau says:
What everybody echoes or in silence passes by as true to-day may turn out to be falsehood tomorrow, mere smoke of opinion, which some had trusted for a cloud that would sprinkle fertilizing rain on their fields. What old people say you cannot do, you try and find that you can. Old deeds for old people, and new deeds for new. Old people did not know enough once, perchance, to fetch fresh fuel to keep the fire a-going; new people put a little dry wood under a pot, and are whirled round the globe with the speed of birds, in a way to kill old people, as the phrase is. Age is no better, hardly so well, qualified for an instructor as youth, for it has not profited so much as it has lost.
My question pertains to the part of the quote that is bolded. What does he mean by this quote?