When its cold I get my 3/4 bib tights out and then put some shorts over when mtb. When its really cold I put some windstopper tights and still put shorts over them. My knees need to be looked after so I keep them warm and take Glucosamine as well (dont know if it works but knee pain is not coming back so I guess it does work for me).
If You have any knee condition or need to look after them than tights/warmers or something like that need to be used.
Back home when it was colder than 12-15 all my club mates were starting to wear longs or 3/4, but again winter is colder than here. Last week was about -15 to -21. Try to wear shorts only when its that cold. 🙂
Scots look like they are cold resistant, never saw so many people wearing short sleeves or just a sweatshirt in winter 😉