[beowulfey]

I would slightly modify your suggestion for 1; it may be easier to gamma irradiate the fecal samples (or something) to kill all microflora instead. It won’t tell us whether the chemical effectors came from the host or the bugs but I would prefer comparing healthy mice than two separate classes of mice.

Another idea I’m curious about would be to preserve fecal samples from a young mouse and transplant back to their older selves to see if that also shows the same effect.

[ce4]

It may not kill everything, eg. this one is known to be highly tolerant to irradiation:

https://en.m.wikipedia.org/wiki/Deinococcus_radiodurans