Problems of gene therapy (increases cancer risk). Ways to insert foreign mRNA into cells (adding special sequences/tags to prevent it from being identified as viral mRNA as cells are basically allergic to it)
GDF11 tried to be used for hypertrophy and sarcopenia - couldn't be repeated in either
The massive difficulty of using CRISPR to target aging-related damage (it's just much better with mononuclear events)
The basic difference between "clear age-related damage" and "entropy-related damage" (e.g. too much/too little RNA, loss of spatial localization [proteins being in places they shouldn't be], loss of stoichiometric regulation of multiprotein complexes)"