Airdrop is a feature available in Lion that allows users to easily share files between Macs without sharing a connection to the same Wi-Fi Network.
It uses PAN to provide for p2p connection (like Bluetooth does).
It may happen that your old mac does not carry a Wi-Fi module capable of supporting this type of network but there’s a known workaround to make this possible.
Fire up your terminal and issue
defaults write com.apple.NetworkBrowser BrowseAllInterfaces 1
Once you did that, relaunch Finder by Opt+Clicking onto the Finder icon in the dock or by issuing
killall Finder
in a terminal shell.
Now you should have your old-fashioned MacBook running both Lion and Airdrop.
Thank you @LorenzoMMariani that pointed this out to my attention.