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The question is:

If i could replace my arm with the arm of someone older than me, would the fact that the cells in the transplanted arm are older cause any specific reaction?

Please handwave all genetic incompatibility issues except for the age of the transplanted organic material.

And let´s assume that the full transplant is possible and goes well.

I remember reading somewhere that the cloned sheep Dolly seemed genetically older because they used an already existing cell in her making. Or is my memory broken?

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    $\begingroup$ As Franz points out, the actual cells in the donor arm won't be appreciably older (in absolute time) than the cells in the recipient body. Do you mean perhaps that the DNA is "older", in that it has shorter telomeres and thus cells are closer to being unable to replicate? $\endgroup$
    – elemtilas
    Jan 28, 2022 at 20:47
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    $\begingroup$ @elemtilas That telomere shortening is also the only relevant aspect of the described situation that I could think of. $\endgroup$
    – Harthag
    Jan 28, 2022 at 23:23
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    $\begingroup$ This question suggests a of lack of research. The single largest complication to transplants is immune systems. Immune systems are not mentioned at all. $\endgroup$
    – Gault Drakkor
    Jan 29, 2022 at 1:02
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    $\begingroup$ @GaultDrakkor -- Yep. Tissue incompatibility is the only real (physical) issue --- the arm is foreign and the recipient body will reject it unless the person is taking a serious immunosuppressive regimen. Even if the DNA in the donor arm cells is 20 or 30 years older than the recipient, the age discrepancy still won't cause any problems. We transplant so many organs now that physical rejection is a known phenomenon. When it comes to parts like arms and hands and possibly even faces, there is the horrible phenomenon of psychological rejection. This is where the recipient can't ... $\endgroup$
    – elemtilas
    Jan 29, 2022 at 4:25
  • $\begingroup$ (cont) ... come to grips with the foreign hand actually being part of his own body. His mind might reject it and he might have it removed. That's a possibility for an arm transplant recipient, but this of course has nothing to do with the age of the donor or of his DNA. $\endgroup$
    – elemtilas
    Jan 29, 2022 at 4:26

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No

Because the cells in Your transplanted arm wouldn't be older than Your own.

Our cells are constantly dying and being replaced by new cells, so the "average cell age" is roughly equal among people (with the exception of newborn babies)

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    $\begingroup$ So teoretically you could replace a 20 year's old arm with the arm of someone in their 80's and there would be no reaction? $\endgroup$
    – mcbecker
    Jan 28, 2022 at 19:28
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    $\begingroup$ I edited the question to cover all kinds of reactions and not "attack" ones. $\endgroup$
    – mcbecker
    Jan 28, 2022 at 19:29
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    $\begingroup$ there would very likely be reactions, but none based on the age of the cells. $\endgroup$
    – Franz Gleichmann
    Jan 28, 2022 at 19:31
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    $\begingroup$ (1) Not all the cells are dying and being replaced with new ones. For example, neurons don't do that. (2) Healthy cells have limited number of division cycles available. Cells which somehow overcome this limitation are called cancer. (3) Some critical tissues cannot be repaired anyway (because it does consist of living cells); in the case of the arm, the hialine cartilage in the tendons and joints falls into this category. $\endgroup$
    – AlexP
    Jan 28, 2022 at 20:06
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    $\begingroup$ Trying to visualize what that 20 year old with the 80 year old arm would look like 60 years later is hurting my imagination. $\endgroup$
    – Harthag
    Jan 28, 2022 at 23:44
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More inflammation and pain.

Older cells have shorter telomeres and other signs of aging. The older cells would as such die more often or fail to work, and so the immune system would have to remove cells from it more often.

This is normal for aging, and wouldn't be a massive issue.

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    $\begingroup$ Also, a greater cancer risk, I think. $\endgroup$
    – wizzwizz4
    Jan 30, 2022 at 0:18
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No:

If we assume an ideal situation, with a person's identical twin (with the identical egg developed years later), then the early twin's arm being cut off and attached to the younger body, there should be no rejection unless an exotic element is added.

The older body may, for example, attack the younger one due to an autoimmune disease. For example, if a person had the genes predisposing them to Celiac disease, and the older arm had some of the cells producing anti-gluten antibodies, those antibodies could then attack the digestive tract of the person getting the donated limb (effectively causing celiac). An old person with a weakened immune system could have a disease such as CLL, and the young body might successfully recognize the cancerous cells as foreign, so then the young body could potentially destroy any (defective) bone marrow in the arm of the old limb. Or the older may harbor an infection that the younger body recognizes as foreign. But these are exceptions, not the rule. I don't know of a lot of Frankenstein-style experiments like this personally, so it's a bit hard to say definitively how differently aged cloned body parts would react for sure.

Older bodies do tend to have fewer stem cells and shorter telomeres, which may help repair damage. But the role of stem cells and telomeres both in aging is a bit controversial, to sat the least. Almost anything involved in tissue growth and regeneration is also at least in part associated with cancer.

What you are thinking of in terms of cloning is that the differentiated tissues, which have gone through many rounds of replication, are both hard to "un-differentiate" into stem cells that can perform all roles of body cells, AND that "older" cells may have short telomeres which protect against the effects of cellular replication. So the Dolly clone was essentially older, and suffered negative effects of age at a much younger age due to this (this is over-simplifying). If you want to know more, read HERE.

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    $\begingroup$ I'm interested in the exceptions. Can you please explain more? $\endgroup$
    – mcbecker
    Jan 28, 2022 at 19:31
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    $\begingroup$ it's worth mentioning: autoimmune diseases and old infections can happen just as well when transplanting from younger to older. (it's just likely that the older body would have those inflictions) $\endgroup$
    – Franz Gleichmann
    Jan 28, 2022 at 19:34
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Organ donors are often older than the recipients.

Consider living donor liver transplantation.
Liver transplantation for pediatric inherited metabolic disorders

Suppose it turns out my new baby has an inherited genetic disease; perhaps maple syrup urine disease. I have a large liver by virtue of my overall largeness. I can easily do without the small left lobe of my liver. That left lobe is as big as my child's whole little liver!

So I donate my left lobe. Baby is cured of the maple syrup urine by my donated liverly awesomeness. Usually parents are the living donors for babies with these issues. I am sore for a while but fine.

I here assert that parents are in all instances considerably older than their children. So baby's new liver is as old as baby's parent! Or even grandparent. That is just fine. If baby grows up with unexplained fondness for Led Zeppelin I will nod knowingly.

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