No hard fast temperature barrier for me. It varies based on sunshine, wind and wind direction, humidity, the planned route, how I feel.
Definitely I put tights on when it's 53F or cooler. Definitely I wear shorts when it's 65F or warmer.
My calves get colder before my knees do then get tight and crampy. My pudgy knees are well-insulated. The knees also swell and get stiff when they are hot and sweaty.
I have several weights of tights that I wear over shorts. Thin running tights, then unpadded cycling light-fleece, thick-fleece, or am-fib/tri-flex as temps drop.
The thin running tights roll up small and pack light. If I think they will be shed I carry an ultralight drawstring backpack to carry them in.
And yes, tights over shorts help minimize laundry. Shorts need to be clean each ride but tights can be worn several times between washes.