Concerning datasets, there are several, depends on Your final purpose.
One of the simplest is output from OSI-SAF, which has info about Sea Ice edge, concentration, type, emissivity and drift. It is derived mostly from polar orbiting meteorological satellites, I guess.
More detailed info can be found on Univ of Illinois pages
Visual images from MODIS satellitescan be handy
But, most detailed info about sea ice conditions can be derived with radar satellites, with commercial options like radarsat being available. From free sources, data from Sentinel-1 is available since late 2014.
So the best info is available from satellites, in-situ measurements do not cover most of the area.
Our knowledge of Arctic sea ice is pretty good IMO, at least since 1979 when satellites started to measure the extent. Understanding the age of sea ice is a bit more tricky, as it can be up to ten, maybe even few more years