55F or higher.
But there are other considerations..
If it is a sunny day I willplay in the high 40's. The sun does warm you up..
Wind..if the wind is blowin 10-15 steady and it is a cloudy overcast day...I will raise my limit to 60.
Concerns are twofold:
I'm 63 and do not want to pull or tear anything in my leg.. I already tore a distal bicept tendon playing and ALTA match in snow flurries.
Secondly, my hands/fingers get cold and I don't get a clean feel for the bevels on my racket.
When people call me in the winter, the first thing I do is check weather.com