I have an old usb printer and I'm wondering if it's possible to use it to print from an Android device.
I'm thinking a micro-usb-to-usb adapter and then just plug it in... Will it work? If not out of the box, how simple is it?
It likely wont. Because two things:
1) Drivers: Android wont know how to operate that printer. All ends here.
2) Even if printer has some Gandalf's spell to be detected by android, AFAIK android doesn't have a mechanism to pass out binary pixel data to printer to print the image.
There are a couple of options out there, here is a good article detailing them -
Everything You Need to Know About Printing From your Android Phone or Tablet
Personally, I've had some luck with my old printer using an OTG cable and the PrintShare app. It seems to have a good library of drivers.
I would plug the USB printer into a nearby computer and enable network sharing, meaning you should be able to find a way to print from your Android; I use Google's cloud print app to print from my my phone to a shared network printer.
As for a direct connection between the printer and the Android, I don't think it's possible.