Straight from Apple's web site:
Minimum requirements for AirPlay Mirroring
For best performance, make sure that your system meets AirPlay Mirroring minimum requirements.
iPhone 4S or iPad 2 (and later) with iOS 5 and later
Apple TV (2nd or 3rd generation) with software version 5.0 or later
An 802.11a, -g or -n wireless network