Well, it's a growth factor. Even IGF1 and growth hormone can rejuvenate old tissue, but they don't make one age any more slowly (though they can make one more robust up to the end).

Do you think that growth factors like these can accelerate aging in the end though? Reductions in growth factor signalling are often associated with increases in longevity, especially since many growth factors increase mTOR signalling (which often results in lowered rates of protein autophagy). I'm not sure how GDF11 would impact mTOR signalling though.

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Edit: Or is it really a growth factor? https://en.wikipedia.org/wiki/GDF11 says that it's a growth differentiation factor.. And that it negatively regulates neurogenesis.. Hmm..