This is caused because you are running a adb other than the one included in the SDK. If on linux check where is the adb binary located
which adb
Expected Output : ANDROID_SDK/platform-tools/adb
If not pointing to ANDROID_SDK/platform-tools/adb then you are running some old version of adb installed on a different location on the machine.
Nothing wrong running adb other than the one provided with SDK but the downside is it is not updated automatically when the android SDK is updated and that's why you running into this out-dated version issue.
The easier fix and to avoid this issue in future rename the older (misleading) adb binary file to something else.
Follow the steps to resolve this issue.
$ which adb
*o/p /usr/bin/adb - (output will depend on your machine)*
$ cd /usr/bin/
$ ls -lt | grep adb
*o/p -rwxr-xr-x 1 root root 160912 Mar 31 2016 adb*
$ sudo mv adb adb_bakup
$ ls -lt | grep adb
o/p -rwxr-xr-x 1 root root 160912 Mar 31 2016 adb_bakup
$ export PATH="/path/to/android_sdk/platform-tools:$PATH"
$ which adb
*o/p <your android sdk dir>/platform-tools/adb* ---> You are all good now