I purchased a home built before 1920. The floors are all hardwood, but have never been refinished. They are in perfect condition and I want to keep them that way. What can I put on them to protect them?

handywoman in the 20's they used primarily wax to finish hw floors but, you may want to consider a nice semi gloss oil modified latex finish. it has the benifits of both oil and latex.
how long are you in the house? you may want to consider hiring a pro floor finisher to do the job (not saying you couldnt)