You can always wait before upgrading. Apple typically continues to support older releases for at least two years after it's general availability. OS X Yosemite (released in October 2014) received its last security update in July 2017. OS X El Capitan (released in September 2015) last received a security update in March 2018 and might be the last update we'll see for this release. In both cases, that's around two and a half years of support.
Edit: macOS Sierra and OS X El Capitan just received another security update (2018-003) on June 1, 2018.