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(I'm not super familiar with biology just curious, so bear with me!)

My understanding:

As people get older (or are very unhealthy) they have accumulated more "bad habits" than others who are younger. The best way I can describe my understanding of this is with the sun. As people get older they have had more time on the planet to be exposed to the sun and are more likely to get skin cancer (which can be exacerbated by tanning and such). Older people are also more likely to have strokes. Usually caused by age-related conditions (hypertension, heart conditions, artery build-up, etc).

Question(s):

To get to the gist of it, why do we get more sick/have more conditions as we age? Is the body just breaking down? are the muscles (like the heart) getting tired? I believe a lot of deterioration is because the cells degrade and don't multiply/do their jobs properly anymore. I've vaguely heard that this can cause damage to DNA and something called "cellular senescence" which I don't fully understand. Is this DNA damage the reason women over 35 are more likely to have babies with birth defects? Because the baby can't form properly with the damaged "instructions"?

Thanks!

edit - I now know more about senescence. This seems to be that we know this deterioration happens, but I'm still missing the why portions. Why do our cells struggle to multiply correctly and function properly as we age? what concept of aging makes this happen? We often explain something happening (like a cell deteriorating in function/health) as a result of aging. But why does more time, mean worse function? Is it just because statistically more time = more opportunity for mistakes? The question about DNA damage and pregnancy still stands.

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    $\begingroup$ Welcome to Biology StackExchange! Generally, it is expected that you do some prior research on the question yourself before posting it here. Ageing and senescence are very complex topics, and there are many different theories trying to explain these phenomena. You can start with a Wiki article on Senescence and familiarize yourself with the basic terms and concepts :) $\endgroup$
    – Domen
    Commented Apr 26 at 8:27
  • $\begingroup$ Thanks for the article! I've tried to do some research, and only really found things about cell division and mutations. I suppose it is hard to research when you don't exactly know what you are searching for haha! Now knowing about Senescence, I'm still left with the same question of why it happens (this just seems to imply we know it happens). I'll be updating the original question accordingly. Thank you! $\endgroup$
    – Rookynote
    Commented Apr 26 at 23:19
  • $\begingroup$ Perhaps your researches need to include some basic evolutionary ideas. The reproductive years are over by the time senescence sets in. The flip-side argument would be: Why would we have evolved to live longer than we do? $\endgroup$
    – Jiminy Cricket.
    Commented Apr 29 at 4:08
  • $\begingroup$ A cell that can divide indeifnetly without end in a body is either a stemm cell or a cancer cell. you want cells to have limited replication in a multicellular organism to prevent the latter. $\endgroup$
    – John
    Commented Apr 29 at 20:41

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Let me start by answering your biggest "why". Ultimately, since human beings are fertile from their mid teens and up until their middle age, there was absolutely no reason for us to live any longer than 40 or 50 while we still lived in the wild. The sad thing is that biologically speaking we COULD have evolved to live for hundreds of years but we simply don't. Deterioration associated with ageing happens because there is nothing to stop it.

Now to get more specific. Human genetic material is LINEAR, which means it has an end (like a shoelace). The ends of our chromosomes do not have any genetic information but as cells divide, the ends get shorter and shorter and eventually we start to lose important genetic code. This is just one of many ways in which our bodies get worse with age. There is no single mechanism behind it. Our immune systems also get weaker for several different reasons. As mutations accumulate you also have and increased risk of cancer.

If you want to learn a bit more on this topic the YouTube channel Kurzgesagt has several different videos on ageing and they specifically touch on senence like you mentioned yourself. If you wanna get really REALLY deep into this then you are going to have to pick up some textbooks on genetics, immunology, and cell biology.

Sources as if any of this is somehow controversial: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5719695/ https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5291468/#:~:text=Beginning%20with%20the%20sixth%20decade,to%20support%20appropriate%20wound%20healing. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5322748/#:~:text=The%20aging%20process%20is%20driven,their%20efficiency%20declines%20with%20age. https://pubmed.ncbi.nlm.nih.gov/11327115/#:~:text=The%20regulation%20of%20telomerase%20activity,cellular%20immortalization%20and%20in%20cancers.

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