Retired people usually use much less resources than young people starting families.

Hmmm -- not so sure.
The thing is when you "retired" you become a tax on the working society. A certain part of the net energy gain has to be spent on sustaining that populace -- health care products and services are the most obvious. The cost of raising a child from 0-17 is about $100K in the US (if you take out the housing cost). The life expectancy of people managed reaching retirement age is about "age+15". If the government is giving out 12K/year for Social Security -- that alone is $180K -- not counting other retirement vehicles and medical liabilities. People might argue that they paid for it during their working years -- but looking at our current accounting of debt and obligations, there is a huge mismatch somewhere, both past and present.

Young people starting a family sure cost resource. However, creating a child becomes a drag on resource because of the world we built it -- not because of "creating a child". Once human learn to live within the mean provided by nature, human will be okie. The birth of a child will be similar to a birth of a whale or a bird -- no real net of cost of anything.