I remember that my first year at LSU I was shocked when everyone (including the coaches) wanted to go inside to the indoor track when the temp hit 65 or so. I had just come from OU the year before where I shoveled 2 feet of snow off the pole vault runways so we could vault.
Now, I’m somewhere in between. 50 degrees is usually about my cutoff but it kinda depends on what the workout is. I’ve had athletes run tempo outside when it’s around 30 degrees. For true speed work though I probably would never try that unless I had some way to stay warm between runs.
Really, there’s more to it than just temp though…for example, what are your alternatives? when was the last time you were able to do _____ activity? would it be safer and better to run on a tight-turned, un-banked, super hard surface, indoor track?