Sad news, especially since he just "retired" from DWTS in November. That beings said, I remarked that he did not look good this past season. He had thinned and for the first time, his age was showing IMO. The cause was bone cancer, so it's difficult to know how long ago he was diagnosed.
One thing I didn't realize is that, according to one article, since he was a judge on both the UK and US-based shows, he flew back and forth between Europe and the US each week throughout the season to film both. Flying back and forth between the US and Europe that often and dealing with the time changes, long-haul fights and jetlag can be taxing even on younger folks, so kudos on him for keeping that kind of schedule into his 70s!
Wonder who they'll now get the play the role of 'brutally honest, crotchety old man' judge on DWTS that he filled so perfectly? And I say that with respect and appreciation. RIP, Len.