Every time Apple upgrades to a new version, new features are added, and vulnerabilities of the old version are patched. Currently, iOS has been updated to version 8.3. In this version, Bluetooth connection issues may arise if users do not know their iPhone's version to fix Bluetooth issues on iOS 8.3.
Similarly, in the case of needing to connect iPhone and Apple Watch, if your iOS version is 8.2 or above, the connection will be very easy. However, if your iOS version is below 8.2, then you definitely won't be able to connect.
If you're using an Android phone, you can easily check the current Android version your phone is running to install many useful apps compatible with different Android versions.
If you're using an Android phone, you can also easily check the current Windows Phone version your phone is running to install many useful apps compatible with various Android versions.