If your Mac’s more than about seven years old, you’re probably not going to be able to upgrade it to Mojave, but at best will be stuck with High Sierra or earlier. One fundamental determinant is your Mac model and its hardware capabilities. Now’s the time to decide what you do next. From this autumn/fall, you’re on your own, and any vulnerabilities which remain in Sierra and its bundled apps will be left unfixed, as they are for all previous versions of macOS. If you’re still running Sierra, the release of Catalina is expected to affect you greatly: if Apple sticks to its longstanding policy, that marks the end of security fixes and support for macOS 10.12.
I appreciate that your mileage may vary considerably, though.
With the exception of one major bug in the scheduling of automatic Time Machine backups, which wasn’t fixed until High Sierra, for a great many Macs and their users it was largely trouble-free. Sierra has to have been one of the best recent major versions of macOS.
