I'm assuming you have a old Magic Mouse and Keyboard.
You need to unpair them from your old MacBook Air first. Then you can pair to your new MacBook.
From the Apple Support site
Make sure your device is powered on.
If the LED is blinking, your device is in Discoverable Mode. If the LED is steady, your device is paired with another item. Unpair the device, turn it off, and start again.
In the menu bar of your Mac, choose the Bluetooth icon, then
choose Open Bluetooth Preferences.
Your Mac scans for available Bluetooth devices.
When your device appears in the Bluetooth Preference Pane, click Pair.
In fact, you don't have to unpair it per se - so long as the old MBA isn't in range (off, works) you should be able to pair it with no problems.