Children are not immediately helpful to GDP. They do not contribute to it, nor do they help stock and bond market returns in any meaningful way; their parents are likely disinvesting to pay their support. Young adults are the driving force in GDP growth; they are the sources of innovation and entrepreneurial spirit. But they are not yet investing; they are overspending against their future human capital. Middle-aged adults are the engine for capital market returns; they are in their prime for income, savings, and investments. And senior citizens contribute to neither GDP growth nor stock and bond market returns; they disinvest to buy goods and services that they no longer produce.
To their credit, they don't offer investment advice on this simple age structure formula. The world is obviously more complex than this, but a country's age structure frames the kinds of policy and investment decisions that governments, private institutions, and private individuals are going to be making.