We cyclo-cross folk do all sorts of strange things to keep wearing shorts...

Try rubbing a warming liniment or heating balm [usually camphor-based] into the exposed skin of your legs and then cover that with a coating of vaseline. That'll keep your legs warm in crummy weather down to about freezing for about 90 minutes. If you don't like the goo and it's not that cold or wet, you can get stick-on camphor plasters to paste onto your knees to keep them warm [or at least to promote circulation that keeps them warm]. You may have wondered how pros like Mario de Clerq stay warm in shorts in February. That's how.

I think the sub-65F [18.5C] warm knees rule is a little over-doing it, to be honest. I feel quite comfortable and have never had problems down to about 14C [57F]. From there down to about 7C [45F], however, I'll always wear kneee warmers, knickers or vaseline, and below that, tights. This morning, I was wearing polypropylene tights at -5C [23F] and quite comfortable.