Well yeah what ryan said. Though it kinda sucks, having a outdated phone like nexus s where it stuck on 4.1 which i think is best firmware anyway according to most people, i am not sure why besides it still works with flash natively, and ouya even use it cause of better bluetooth compatibiliy i think. Other than that, if your phone hasn't updated in least a year, time for new one cause official ota updates are no more supported.
Truth is each android phone provider has their own specific updates, is based on google official updates but then modified to work with your specific device. Meaning if you see others have 7.0 and you still on 6.1 is up to who ever makes the product to release updates when they can do so. I have my shield devices on andorid 6.1 which is the android version but the nvidia firmware is 3.1 which supports 6.1, kinda confusing, but is how it is.
Think of it as this, If a new android device comes out today, it would probably support android 7.0 OS under system settings it should say so. But since is first release it should say system build version 1.0
7.0 = latest google official releasae
1.0 = system manufacture modified release
Anyway, like ryan said, unless someone create custom roms based on your phone or device that supports newer android os, most likely not official. You would also need to have a unlocked boot loader, which doing so will format the system memory if you haven't done it yet, root your phone if you know how and what you need it for. But as for consumer use is basically better to just get a newer product.