Can I downgrade macOS to OSX Mavericks if the earliest OS that came with my Mac is macOS Sierra?
If your Mac originally shipped with Mavericks, then yes, you can install Mavericks on it, regardless of whether when you bought it, it had Sierra on it.

Otherwise, no.  Think about it.  If your Mac was released after Mavericks, Mavericks won't have hardware drivers for it.