Most old 12 speeds which today we'd call 6 speeds or a 2 x 6, were freewheel and wouldn't be able to use this cassette. But any that have a 11 cog on them might could use this cassette. Most freewheels I ever have seen only had a 14 tooth on them for the smallest tooth count.

I've never seen a bike with a six speed cassette. I know they exist, but they were rare and not anywhere for me to see.