You can make any USB printer that has Linux drivers AirPrint-compatible by using a Raspberry Pi. There are loads of guides out there.
This same method will also let you use your printer on computers with newer versions of the OS that may no longer have drivers for your printer available (just set the computer up to use a network printer via the Pi with a generic postscript driver. The Pi handles the rest).
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http://support.hp.com/us-en/document/c02623193
They already did in the 80's and 90's, they had loads of them!
This same method will also let you use your printer on computers with newer versions of the OS that may no longer have drivers for your printer available (just set the computer up to use a network printer via the Pi with a generic postscript driver. The Pi handles the rest).
Same here