The Notes app on the recent iOS 9 versions has more function (pictures, scetches, etc.) and notes will no longer sync via iCloud with iOS 7 or earlier, and maybe not iOS 8. What happens is that the iOS 9 notes will sync with other iOS 9 (and maybe iOS 8) devices, but older iOS devices (older gen iPods, older gen iPhones) will only sync with each other. This restriction exists even though all devices are signed onto the same iCloud ID. iCloud seems to be creating two separate Notes instances within the cloud.
If you want to sync simple text only notes between all iOS versions you can create a Google/Gmail account and turn on Notes sync in that account for all devices. The Google/Gmail group will show as a separate account on each device's Notes app and are shareable, i.e they will sync across all versions of iOS. You could probably use a Yahoo account to enable Notes sync but I haven't tried it.
It is not possible to sync the iCloud notes easily, but you can copy important ones from iCloud and paste them to the Google or Yahoo Notes account within the Notes app.
This information is based on a call and discussion with a senior Apple Support rep. There are no plans to support the new Notes function and syncing on the older IOS versions.