A lot has been said and published about the energy and material impact of houses. What usually stays out of sight is the inhabitant. And I do not mean his behavior, but his ‘being’ , his need over the years for shelter. The environmental impact is in the end a result of a combination of technology, affluence and population (growth) . Paul Ehrlich already symbolized this in his equation I=PAT
affluence and technology get the most attention, while attention for population is mainly addressing the total amount of people waking on this earth.  Now population growth can be by absolute growth, but growth can also come from the life expectation from the people: The total years of demand for shelter: Usually we calculate with an average, but we all get older, the life expectation increases. Which implies that reducing environmental impact is not related to a fixed target, but is a moving target.
Say if we get 10 years older on average, The impact of the house we live in, for an extra 10 years remains the same for the physical unit a house is. However the impact of the inhabitant, his claim on shelter, increases with 10 years. In other words, the house will not come available for newcomers in the housing market, a 10 year gap unfolds in which no houses come available, and more new houses will have to be built, resulting in more environmental impact. By definition. At least, with current fertility rates, pushing new comers into the market at the same speed as before.
If we all grow older, fertility rates should go down to remain the same level of total m2 shelter need per year . ( and for the other resources as well.) Or we reduce m2 ’s of shelter per person.
So if people get on average older and require longer time of shelter, the environmental impact increases: which is probably one of the reasons we are not so successful in reducing our footprint: all achievements are offset by the our healthy life.
On top of all this we see a trend that houses might become a commodity: a product with a shorter lifespan, which reduces the available amount of m2 shelter , and increases the need for construction.  Our ever increasing attempts to reduce environmental impact from housing is threatened from two sides….Not to mention the fact that 1) houses have become larger over the years *, 2) the embodied impacts form housing increases due to the use of more ‘high tech’ materials ( requiring more energy during processing) and 3) the increase of population in the absolute sense, globally very hard, ( around 155 per minute, over 20.000 pr day  ) , in the Netherlands mainly immigration related.
And if thats not enough, we have the medical science. Which makes huge efforts to prolong our lives, on top of the above trends: not only fighting diseases, but also fighting aging. There are serious signals that we all could become 120 years during this century. Which will disrupt things enormously: free fall of houses will be delayed, and require an extra 50% of houses ( from 80 years average to 120 years average housing need) , and a s a result 50% more impact from construction. And thats only for housing. ( there are even signals that aging could be further delayed , and even stopped. Then things get complicated….)
This all is connected to a phenomenon that gets very little attention, the age at which women are giving birth. Not the fact itself of getting children, the fertility discussion, but the moment during their lifetime that the first child is born. : Women getting children at older ages is good for environment! Just see this example: :
Imagine a century, in which women get children at the age of 20 : In that century 5 generations are added. Or imagine getting children at the age of 33: in that case 3 generations are added : Simultaneously living, and of course in the first case a much higher resource load/need grows during that century. Or more precise: if people on average get 75 years old and bring 2 children to the world, 1 boy and 1 girl on average, the first case counts for more or less 720 life years per century, the second case around 470 per century, to feed and house. Both having 2 children! Delaying childbirths is a effective way to reduce resource loads…
If you think of this freely, then the following could be the best natural strategy: You have to raise children , to learn to cope with surviving. You need children to take care of you if your skills to survive decline. Say both is 20 years: 20 years of eduction, and 20 years of aging with need for care. In that case the best social strategy is to get children not if you are young, but 40 years before you die. If people grow older, so shifts the optimal birth giving age, to reduce combined environmental impact.
This is not that special, in history optimal timing has changed as well, in the early middle ages, people married at early teenager age, birth giving very soon after, and dying on average around 40. And the age has already been shifting recently as well , In the Netherlands people marry relative late, and child birth giving ages is increasing .  Maybe this is a natural but unconsciousness drive to relief the environment? Somewhere some genes might still work in favor of survival?
There is lesson to be learned here: if a society is doing well, with growing welfare for each inhabitant, and supposing or momentarily requiring that each partial system has to deal with its own resources, not eating I on others, its highly necessary that efficiency in resource use increases, at least at the speed by which life expectation increases. And thats only to remain stable in our impacts, not to improve.
I don’t have calculations, but you might expects that increasing life expectancy might outpace our reducing impact strategies, seeing how slow we succeed in the latter. Or Medical innovation goes faster then technical/environmental innovation…. We are loosing on our side…
In that case, indeed, the birth giving age should be raised, or, and thats another escape route: The replacement ratio has even further to decline. 2,1 children per family is even too much then…
 zie eerder artikel: http://ronaldrovers.nl/?p=75
 population clock: http://www.worldometers.info/world-population/