Getting off topic now a little, but seems like you've gotten some good answers to the main question.
I remember hearing on a TV show a long time ago, I think it was Hank Parker, that 55 degrees was the "magic" temperature for throwing topwaters. I think he was specifically talking about a buzzbait. That seems to fit with my experience in the spring when the water is warming.
That said, I've caught them down to 50 degrees here in MN throwing a buzzbait and covering water in the fall. That's a pretty common occurrence on some bodies of water where the fish get shallow and spread out at that time of year. You've got to cover water to find them, and the buzzbait does as well as any technique out there at covering water. Maybe more interesting is that I actually caught 2 smallmouth on the Mississippi Pool 4 on a Spook in 47-48 degree water two or three falls ago. To be fair, they were busting shad and I threw at them when I saw them come up so I suppose that may have been sort of a special scenario, but it was high 40s nevertheless. Maybe the more interesting thing is that I had a Spook tied on when fishing water that cold. Not sure what I was thinking beforehand, but it worked out!
The other thing is that I think the further north you get, the more "tough" the fish are when it comes to cold weather. Fish here in MN seem to be bothered by the cold a lot less than fish where I grew up in WV, and I'm sure that holds true comparing it to places further south. It holds across species, too - bass, trout, crappies, sunfish, etc. It's not really surprising when you think about it. After all, humans in MN are a lot less bothered by the cold than humans further south, and we get the luxury of living in climate controlled environments for the most part!