It's not what you think.
The cornucopia is a frequent symbol of the season. Most people who talk about the coming Age of Plenty believe that the next high-tech breakthrough will solve all our problems; they are sometimes called cornucopians. I do not share their belief.
According to Wiktionary, plenty means "A sufficient quantity. More than enough." This concept of "plenty" has gotten warped over the past century, which I like to call the Age of Profligacy.
Back in my great-grandparents day, they would say "We have plenty," or "We have enough." By this they would mean that they had a roof over their heads, clothes on their backs, enough food not to go hungry, etc. In other words, all their material needs were met.
The problem came in when we started using materials goods for purposes other than meeting our material needs. Take food as an example. We need energy to live and to do things, and we need basic building blocks to repair and rebuild our bodies. Food satisfies those needs. When we start using food for other purposes, such as to entertain, to stimulate, to relax, we can easily eat more than what we need. In extreme cases of overindulgence we can end up as half-ton people.
With our economy predicated on perpetually consuming more, marketers have been encouraging us to overindulge. They specifically try to simultaneously make us feel somehow inadequate and to convince us that buying whatever they are selling will solve that inadequacy.
This never was good for people's bodies or spirits. However, while resources were extremely plentiful compared with demand, the system worked as designed. But that hasn't been true for two generations now. Starting with the Arab oil embargo in the early 1970s, availability of resources has been a limiting factor in our economy. As a free market economy is supposed to work, lack of availability is evidenced by increasing prices. This allows for smooth adjustments -- sometimes too smooth. Only when government tries to force prices down do we get other means of restriction, such as long lines waiting for gas.
Eventually there comes a price where people voluntarily cut back. In 2008 Americans did this with gasoline when it reached $4 a gallon. Prices went down when a falling economy pushed demand back down. Once prices went back down, people resumed their profligate ways.
I expect this process will be repeated many times over the coming decades. But each time it happens, more people will step back and look at what they really need and come to the conclusion, "We have enough." When the majority of people have done that, the Age of Plenty will have arrived.
That will be a major milestone on the Long Ascent.