From my research, it seems that macOS transitioned from 32-bit x86 to 64-bit x64 across the following versions:
Kernel App Support 10.4 Tiger x86 x86 10.5 Leopard x86 x86/x64 10.6 Snow Leopard x86/x64 x86/x64 10.7 Lion x86/x64 x86/x64 10.8 Mountain Lion x64 x86/x64 10.9 - 10.12 ... x64 x86/x64
- Support for x64 apps was added in 10.5.
- A 64-bit kernel was introduced in 10.6, and the 32-bit kernel was removed in 10.8.
- It is possible to run 32-bit apps with a 64-bit kernel and (unlike Linux and Windows) 64-bit apps with a 32-bit kernel.
Is this accurate?