How old are Janus and Epimetheus, and are they believed to have formed out of a collision? (I think would explain their shared orbit).
Not the age of their elements, which should be similar to the age of the solar system, but the age of their surface and formation.
Cassini told us that Saturn's rings formed relatively recently, 100 or 200 million years ago. Are there similar aging techniques that can be applied to the surface of icy moons and as that been done? (related, this type of aging technique should apply to Pluto and Charon too, because they are believed to have formed by collision).
Edit: I came across this which says:
The extensive cratering indicates that Epimetheus must be quite old. Janus and Epimetheus may have formed from a disruption of a single parent to form co-orbital satellites, but if this is the case the disruption must have happened early in the history of the satellite system. From its very low density and relatively high albedo, it seems likely that Epimetheus is a very porous icy body. There is a lot of uncertainty in these values, however, and so this remains to be confirmed.
So, significant uncertainty but probably quite old, early in the history of the satellite system, which would be fairly early in solar-system formation. Much older than Saturn's rings.
I'll leave this question open in case anyone has better info, the above being from Wikipedia, though they reference a NASA article.