How old are Janus and Epimetheus? 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.[4] 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.
You are correct that the main way to determine planetary surface ages is through crater dating. This comes in two versions, relative and absolute dating. Both require counting the number of craters per area and so you need good resolution imaging since the number of impactors and craters follow a power-law (more small impactors/craters than large). It then basically boils down to older surfaces have had a longer time to be hit by more impactors and so there will be more craters per unit area.

With relative dating, all you can do is put different surfaces in order of amount of craters and say "surface A is older than surface B as it has more craters". If you have another method of dating, then you may be able to say where on the timeline a surface may be but you would still only be able to "surface A is older than surface B (dated to be 250 Million years)" but you don't know much older.

If you have an idea of the rate of cratering bodies and the rate of crater formation in the system you are trying to study, then you can perform absolute dating. This allows the cratering rates to be compared across different bodies. This has been done for the inner Solar System (Mercury, Moon, Mars) but is tricky where the surface can be altered (weather, lava flows etc). There are also problems with crater saturation with very old surfaces where new craters overlap and overwrite older ones and issues with understanding the lifetimes of the craters and what processes can fill-in and erase craters and how long this process takes.

In the case of the Saturn system, determining the number of potential impactors and performing absolute dating is likely to be pretty tricky as the environment is dynamically complex. There are a lot of large Moons and you also have Jupiter and the other gas giants planets affecting things. In addition, the positions of the giant planets are thought to have changed dramatically around 500 Mya after formation (the Grand Tack model and hypothesis).

The images of the Pluto and Charon system returned by New Horizons have also shown some of the difficulties with crater counting (blog article on this by a member of the New Horizons team and crater dating expert). Some parts of the system such as Charon appear to be very old and have large numbers of craters; other areas such as Sputnik Planitia appear to be very young and no-one can find any craters of any size anywhere on that region.