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Ok, this may be just wild nonsense and I have to warn that my knowledge on the subject is limited.

I was reading about how in some countries people choose to have children later in life. After that this question came to my mind?

If human couples chose to have children later in life, lets say 40s 50s - disregarding keeping an younger self gamete preserved - wouldn't this result in the people less resistant to the effect of aging being unable to reproduce, creating a more longevous generation? thus enabling future generations to reproduce even later lenghtening the average lifespan of humans?

ps: After reading this sounded a little too eugenic, by no means I'm trying to make this an legitimate theory or suggest an experiment, it's just a doubt - maybe good for some science fiction story...

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    $\begingroup$ Could you clarify your third paragraph? I think I know what you are asking but I'm not positive. $\endgroup$ – Michael S Taylor Sep 24 '14 at 3:48
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    $\begingroup$ Are you suggesting that women store eggs while young to use post menopause? Becuase that would probably select for women who make eggs that handle IVF well, rather than for longer life. $\endgroup$ – user137 Sep 24 '14 at 6:31
  • $\begingroup$ Are you asking if there is a selective pressure to reproduce later in life? $\endgroup$ – James Sep 24 '14 at 15:01
  • $\begingroup$ @Jonathan Can you give us your opinion about the discussion in the comments below Mike Taylor's answer? $\endgroup$ – Remi.b Sep 24 '14 at 20:23
  • $\begingroup$ @MikeTaylor I'm not sure on how to clarify? I was wondering if the choice of reproducing later in life, allowing only those who were "fitter" at older ages to have children, would result on people that aged "better" and would make these people live longer... I know that many aspects have to be taken out of the picture, like drugs, gamete preservation , the whole humanity having children later. It's just a wild hypothesis . $\endgroup$ – Jonathan dos Santos Sep 25 '14 at 0:13
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Many factors have to be considered. First, although more women are delaying pregnancy until after the age of 35, the overall percentage of women doing so remains very small, as shown by this figure from this New York Times article. Only about 9% of pregnancies in the United States (for 2008) were in women over the age of 35. Most pregnancies still occur in younger women. I suspect this is especially true when considered globally.

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Getting pregnant later in life is more difficult, is more likely to result in miscarriage, and is more likely to result in some types of birth defects. The issues are discussed by the Mayo Clinic, WebMD, and summarized by Parenting Magazine. On average, individuals with birth defects are themselves less likely to reproduce (Skjaerven 1999).

For natural selection to favor a particular trait (such as later pregnancy), the individuals with the trait must have higher relative fitness, meaning they produce more offspring on average compared to other individuals. However, relatively few women have late-age pregnancies, more of the offspring are likely to have birth defects, and those offspring are themselves less likely to reproduce. In other words, on average, late-age pregnancies would have reduced fitness compared to individuals that get pregnant at earlier ages.

Most importantly, though, the decision to wait until later in life is a conscious choice, not a genetic trait. Natural selection can only work on genetic variation associated with particular traits. If the choice for delayed pregnancy has no genetic basis, then it is invisible to natural selection, and so cannot evolve.

Citation

Skjaerven, R. et al. 1999. A population-based study of survival and childbearing among female subjects with birth defects and the risk of recurrence in their children. The New England Journal of Medicine 340: 1057-1062.

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    $\begingroup$ The OP may have thought (and he would have been right) that if we delay reproduction, then deleterious alleles acting late in life will be purged yielding to a longer lifespan. This post is probably of interest to the OP. $\endgroup$ – Remi.b Sep 24 '14 at 13:32
  • $\begingroup$ @Remi.b Yes, if all or most individuals delayed reproduction but I do not think that is likely to happen. Most individuals reproduce early so the late-acting deleterious alleles would remain in the population, sensu the mutation accumulation hypothesis in GriffinEvo's answer in the post you link to. There would also have to be selection to remove the other deleterious effects of late-age pregnancies that potentially reduce fitness independent of life span. $\endgroup$ – Michael S Taylor Sep 24 '14 at 13:48
  • $\begingroup$ The OP is asking "If human couples chose to have children later in life[..]". He therefore does not ask whether it is likely that individuals suddenly change their habit and start to reproduce later in life, he is asking how will senescence evolve if we assume that individuals reproduce later in life. I agree that the 3rd paragraph is kinda unclear. $\endgroup$ – Remi.b Sep 24 '14 at 15:01
  • $\begingroup$ @Remi.b I suspect you are correct that it may be a more theoretical question. I can delete or change my answer pending clarification from the OP. $\endgroup$ – Michael S Taylor Sep 24 '14 at 15:57
  • $\begingroup$ Supposing I understood the question correctly, don't delete it anyway, it is always good to have some insights about the validity of the assumption $\endgroup$ – Remi.b Sep 24 '14 at 16:05
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You will appreciate reading this post and have a look to the wikipedia article on the evolution of aging. I make below a quick summary and apply it to your question but I think it is worth looking at the other post on aging.

In short: Theory of aging

Imagine a deleterious (=negative) allele (=variant of a gene) that is expressed throughout the lifetime. This allele (by definition of a deleterious allele) will decrease the reproductive success of the carrier. Now consider another allele who is expressed only late in life, maybe after the first reproduction but before the second reproduction. This allele will decrease the reproductive success of the carrier but by a lower magnitude that did the allele that was acting throughout the lifetime so that it is more likely for this second allele to rise in frequency in the population to the point of becoming fixed (=reaching a frequency of 100%). In other words, natural selection is less strong on alleles acting late in life. i.e. deleterious alleles acting late in life are more likely to get fixed resulting in the expression of many disease late in life. In the worst case, we can talk about alleles that are expressed after the time of the last reproduction. These alleles are not at all selected against (I make abstraction of kin selection here). Therefore, we should not be able to live long after the age of reproduction. non-reproductive grandmothers is kind of an exception and is explained by kin selection but I don't want to go into these details. Also, today's medicine explain why we live so long even after the last reproduction. For example articulation related disease that we experience at older age is often caused by this kind of alleles that are expressed only late in life. You can also consider alleles that are beneficial early in life but detrimental at the end of the life. THeir probability of fixation is even greater and these alleles have already been observed in nature. For more info about the evolution of aging, have a look to this post.

What happen if human decide to delay their reproduction age

So, what happen if suddenly all humans decide to reproduce later. Well, this will increase the selection against alleles that are expressed late in life and in the long run this will increase human lifespan (or at least it will increase the time during which humans are healthy). So yes, your hypothesis is right. However, as a sidenote, medicine may mess things up in the story!

Is it plausible that humans will start delaying their reproduction age?

For consideration of whether it is likely that the human population decide to delay their reproduction age, see @MikeTaylor's answer.

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