The following chart showing U.S. population projections for the 20-39 age group for the next several decades is incredibly bullish:
This chart is especially bullish because of the large increase in the 30-39 year old population that is projected to occur over the next decade. People in their 30s are in their working prime and they tend to get married and start families at a significantly higher rate than other age groups. Household formation results in home buying, housing construction, credit growth, investment, babies (which lead to increased consumption) etc. All bullish stuff for the economy.
The bearish counterbalancing argument is that the baby boomers (the most powerful economic generation in U.S. history) are retiring and the oldest baby boomers are already beginning to tap into retirement accounts and turning into net sellers of investment assets as opposed to the massive net buyers they have been for decades.