They would have to depend on existing wifi network, which is cumbersome. AirDrop creates it's own peer-to-peer, direct Wifi network. And for that, you need dedicated hardware that simply isn't present in older devices.
So no, not possible. That's why there's no Airdrop capability between an iDevice and Macs (although given the macs are usually connected to existing network, it would make a little more sense).