I've largely switched back to trousers from shorts. I prefer shorts, but I remember reading somewhere that riding in the cold is very bad for knees (blood supply to knee caps are low compared to amount of exposure to cold air received on a bike). I think the saying was "under 20 degrees? Cover the knees". No idea whether it is a myth, but I roughly stick to it and change to trousers earlier than most.
My neighbour (late 50s) had a heart attack recently and is recovering well. He's been cycling to aid his recovery, but was told that while cycling was very good, he should avoid cold weather. Apparently, colder weather makes the heart work harder. I would have thought it was the opposite - surely the heart works harder in warmer weather to dissipate the heat quicker? I've no idea! Maybe it is about extremes.