Bluetooth MIDI requires the use of Bluetooth LE or Bluetooth Low Energy. This technology is part of Bluetooth 4.0 which requires specific hardware that accommodates that technology. The original iPad 1 released in 2010 and the second-generation iPad 2 released in 2011 only have hardware that supports Bluetooth 2.1 which means you cannot run Bluetooth MIDI on these devices. The third-generation iPad 3 and the iPhone 4s and newer devices support Bluetooth 4.0 and higher meaning that you can use Bluetooth MIDI with nearly every device that OnSong can run on.
Keep in mind that naming conventions used by Apple can make these designations confusing. For instance, the iPad Air 2 is much newer and supports Bluetooth MIDI. Any Apple device made in 2012 and newer should support Bluetooth MIDI.